Just because a machine will run with a particular product doesn’t mean that product is optimal for the application. Most lubricant mis-specifications don’t lead to sudden and catastrophic failure, but rather they shorten the average life of the lubricated components and, thus, go unnoticed.
With hydraulics, there are two primary considerations – the viscosity grade and the hydraulic oil type. These specifications are typically determined by the type of hydraulic pump employed in the system, operating temperature and the system’s operating pressure. But it doesn’t stop there. Other items for consideration are: base oil type, overall lubricant quality and performance properties. A system’s requirements for these items can vary dramatically based on the operating environment, the type of machine for which the unit is employed and many other variables.
Selecting the best product for your system requires that you collect and utilize all available information.
Pumps and Viscosity Requirements
Let’s start by outlining the No. 1 lubricant selection criteria: pump design types and their required viscosity grades. There are three major design types of pumps used in hydraulic systems: vane, piston and gear (internal and external). Each of these pump designs are deployed for a certain performance task and operation. Each pump type must be treated on a case-by-case basis for lubricant selection.
Vane: The design of a vane pump is exactly what its name depicts. Inside the pump, there are rotors with slots mounted to a shaft that is spinning eccentrically to a cam ring. As the rotors and vanes spin within the cam ring, the vanes become worn due to the internal contact between the two contacting surfaces. For this reason, these pumps are typically more expensive to maintain, but they are very good at maintaining steady flow. Vane pumps typically require a viscosity range of 14 to 160 centistokes (cSt) at operating temperatures.
Piston: Piston pumps are your typical, middle-of-the-road hydraulic pump, and are more durable in design and operation than a vane pump. They can produce much higher operating pressures – up to 6,000 psi. The typical viscosity range for piston pumps is 10 to 160 cSt at operating temperatures.
Gear: Gear pumps are typically the most inefficient of the three pump types, but are more agreeable with larger amounts of contamination. Gear pumps operate by pressurizing the fluid between the trapped air volume of the meshing teeth of a gear set and the inside wall of the gear housing, then expelling that fluid. The two main types of gear pumps are internal and external.
Internal gear pumps offer a wide range of viscosity choices, the highest of which can be up to 2,200 cSt. This pump type offers good efficiency and quiet operation, and can produce pressures from 3,000 to 3,500 psi.
External gear pumps are less efficient than their counterpart, but have some advantages. They offer ease of maintenance, steady flow, and are less expensive to buy and repair. As with the internal gear pump, these pumps can produce pressures ranging from 3,000 to 3,500 psi, but their viscosity range is limited to 300 cSt.
Fluid’s Roles and Makeup
Hydraulic fluid has many roles in the smooth operation of a well-balanced and designed system. These roles range from a heat transfer medium, power transfer medium and a lubrication medium. The chemical makeup of a hydraulic fluid can take many forms when selecting it for specific applications. It can range from full synthetic (to handle drastic temperature and pressure swings and reduce the rate of oxidation) to water-based fluids used in applications where there is a risk of fire and are desired for their high water content.
A full synthetic fluid is a man-made chain of molecules that are precisely arranged to provide excellent fluid stability, lubricity and other performance-enhancing characteristics. These fluids are great choices where high or low temperatures are present and/or high pressures are required. There are some disadvantages to these fluids, including: high cost, toxicity and potential incompatibility with certain seal materials.
A petroleum fluid is a more common fluid, and is made by refining crude to a desired level to achieve better lubricant performance with the addition of additives, which range from anti-wear (AW), rust and oxidation inhibitors (RO), and viscosity index (VI) improvers. These fluids offer a lower cost alternative to synthetics and can be very comparable in performance when certain additive packages are included.
Water-based fluids are the least common of the fluid types. These fluids are typically needed where there is a high probability of fire. They are more expensive than petroleum but less expensive than synthetics. While they offer good fire protection, they do lack wear-protection abilities.
Application-Based Selection
Application should be the most critical attribute when selecting a hydraulic fluid in order to ensure the system’s ability to function properly and attain long life. When selecting a hydraulic fluid, it is very critical to determine the system’s needs: viscosity, additives, operation, etc.
For example, take a large dump truck that is constantly in the rain, encounters high particle contamination from road debris and leaks 10 percent of its sump volume in two days. There is no need to buy or use the most expensive fluid with the best additive package simply because of the associated cost of replenishment and the inherent lack of maintenance. On the other hand, you have a very clean, critical and highly loaded system that is maintained properly and used to its full potential. You may want to use a more premium product, such as a highly refined petroleum-based fluid with an AW or RO additive package or even a full synthetic fluid.
As far as the viscosity of the fluid is concerned, this should be determined by the pump type as previously discussed. Not having the correct viscosity for the application will dramatically reduce the average life of the pump and system, thereby directly reducing its reliability and production. When selecting the appropriate viscosity grade, look for the optimum viscosity required by the pump. This can be determined by collecting data from the pump OEM, actual operating temperature of the pump, and the lubricant properties referenced to the ISO grading system at 40 and 100 degrees Celsius.
Check the operating temperature of the pump and see if it falls between the temperature ranges of the lubricant in question. If not, you may need to increase or decrease the viscosity of the lubricant to achieve the desired, optimum viscosity.
As you can see, selecting the proper hydraulic fluid for the application is not a hard task, but it does require time to research the application, determine the resulting cost and decide which fluid type is best.
You can spend more or less money than is needed simply by not educating yourself on proper lubricant selection techniques. To practice good lubricant selection is to practice great machine performance!
How do you know if you’re using the Right Hydraulic Oil?
Application-Based Selection
Application-based selection techniques are the reality checks to make sure all of the time spent selecting the proper viscosity, additives, etc., was not wasted by simply ignoring the application’s requirements and operating conditions. Just following OEM specifications will not be enough to ensure that the correct hydraulic oil is selected; these are typically for best-case scenarios. Ignoring these reality checks will most often still lead to failures down the road. They may not be as rapid as selecting the wrong viscosity, but they will eventually happen; therefore, application and operating factors should be taken seriously.
Selecting Hydraulic Fluids – How it Impacts Operating Costs
Every operations manager is challenged to find ways to reduce operating costs while improving productivity. This objective places an increasing strain on staff and equipment. Is it possible that simply changing hydraulic fluids to a better quality hydraulic fluid and adding GG Friction Antidote can result in a significant reduction in energy costs and an improvement in equipment output? Yes! GG Friction Antidote offers an opportunity to reduce costs and improve hydraulic performance. When the heat is up your hydraulic fluid will lose its effectiveness.
It is easy to conclude that fluid viscometrics (viscosity grade, viscosity index, and stability in high-pressure applications) are key in optimizing pump efficiency. Low pump efficiency results in higher energy costs (fuel or electricity), reduced power output and slower response time. With conventional hydraulic fluids, the hydraulic system performance suffers at or near maximum operating temperature. High temperatures “thin out” or reduce the hydraulic fluid’s resistance to flow (viscosity), leaving the vanes or pistons of the hydraulic pump unable to push an adequate amount of fluid. The fluid recirculates, creating what’s known as “internal leakage”. The more fluid that recirculates, the higher the temperatures become within the system because of the additional frictional heat that’s created. This leads to a downward spiral in performance which causes the equipment operator to “step on the gas pedal” to maintain performance.
Using more fuel to do the same amount of work is inefficient and expensive. Things get worse as the hydraulic system becomes progressively less responsive and work productivity falls significantly. When the hydraulic system goes down, so do the hydraulic-driven tools that it powers. Stress increases for hydraulic system components such as pumps, hoses, and seals – resulting in reduced service life. Not surprisingly, the service life is also cut short for the system’s conventional hydraulic fluid, which began the downward spiral in response to high operating temperatures.
GG Friction Antidote treated hydraulic fluids retain viscosity in the face of high operating temperatures when conventional fluids grow weak. As a result pump performance remains stable, fluid temperature is kept under control, overall stress on the system is kept in check, wear and tear on hydraulic system components is kept extended way beyond its service lifecycle and fuel consumption is significantly less than that of a non GG Friction Antidote treated conventional hydraulic fluid. The greater the stress placed on hydraulic equipment the higher the energy consumption. The combination of GG Friction Antidote and a high quality hydraulic fluid enables higher flow rate at peak operating loads and lower energy consumption.
Injection Molding Machines
All companies that use injection molding machines are in constant competition for low unit operating costs. Many steps have already been taken in terms of design. For example, machine manufacturers offer special energy-saving equipment to reduce the power consumption of the machines even further. In general, newer machines have already been pushed a long way in their design with regard to energy-saving measures.
Environmental audits, which are linked to the EU Directive 2012/27/EU (which calls for a 20% reduction in primary energy consumption by 2020 in the EU economic area), put pressure on the manufacturing industry. Companies are reducing their energy consumption with environmental measures such as solar panels, heat recovery systems, or systematically switching off tools during idling phases. The imagination knows no limits but a shrinking marginal benefit is looming here.
One simple and clever step that can be taken is to change the hydraulic fluid in injection molding machines from inefficient to GG Friction Antidote treated hydraulic fluid. The range of injection molding machines that could benefit from fluids treated with GG Friction Antidote is therefore, quite simply, unlimited.
Fluids with constant viscosity ensure greater efficiency and less energy consumption – whether in construction equipment like excavators, wheel loaders, skid steer loaders and backhoes – in mining equipment like screening plants and dump trucks or – in industrial equipment like injection molding machines, robotics and all other types of stationary equipment using hydraulics. Simply adding GG Friction Antidote to a high quality hydraulic fluid allows nearly any hydraulically powered machine to operate more efficiently, dynamically, and accurately.
The use of GG Friction Antidote treated hydraulic fluids pays for itself in a relatively short time, most notably in extremely demanding applications, while also ensuring higher returns with lower energy consumption and reduced emissions. On its way from the tank to the pump, the fluid has to pass several bottlenecks like valves. Friction within the fluid itself and friction on the walls creates heat. Compression in the pump creates more heat. The heat reduces the viscosity. GG Friction Antidote treated hydraulic fluids reduce the friction, thus the temperature is reduced.
Ideally the viscosity in the pump doesn’t get too low to maintain power. For this reason, a fluid with a higher viscosity is normally used because it heats up and becomes thinner on its way to the pump, thus requiring more power for transportation. GG Friction Antidote treated hydraulic fluids keep their viscosity over a broad temperature range. So you save energy, enhance performance and protect the equipment.
Fluid Friction
Incorporating GG Friction Antidote in any hydraulic fluid improves hydraulic operation efficiency, reduces hydraulic temperatures and extends the lifecycle of hydraulic components, fluids and fuel efficiency. The use of GG Friction Antidote pays for itself in just a relatively short period. Fluid friction is the resistance to an object’s motion through a liquid or gas. When the motion is occurring in a liquid, it is referred to as viscous resistance. Resistance to an object moving through a gas, such as air, is termed air friction. The tendency of the liquid to resist flow, i.e., its degree of viscosity, is another important factor. Fluid friction is affected by increased velocities, GG Friction Antidote releases ions that are friction activated in all fluid carriers. When you use GG Friction Antidote in your fluid carrier, energy that would have resulted in hydromechanical and volumetric frictional losses within a hydraulic component is converted into useful energy required to generate optimum fluid flow.
Pump Efficiency
Every type of hydraulic pump and motor is designed to have a small amount of internal fluid leakage or recycle. This fluid is essential because it forms a lubricating film between the moving parts which prevents wear. If pumps and motors are operated at optimum temperature and pressure conditions, the amount of leakage is minimal and the pump can operate with greater than 90 percent efficiency. However, hard-working equipment is often placed under significant stress, resulting in high oil operating temperature. As the temperature of the oil increases, the oil viscosity drops, and higher levels of internal leakage occur. It is not uncommon for mobile equipment hydraulic systems to produce sustained operating temperatures in the 80 to 100°C (176 to 212°F) range, with temperature spikes ranging between 110 to 130°C (230 to 266°F). Oil temperatures greater than 60°C (140°F) decrease viscosities enough to have an appreciable negative impact on volumetric pump efficiency. It is common to find pumps operating at 50 to 60 percent volumetric efficiency when the oil temperature increases to 100°C (212°F). When a pump works at 60 percent efficiency, 40 percent of the input energy is wasted and converted into heat instead of work.
There are two elements of hydraulic efficiency: volumetric efficiency and hydromechanical efficiency. Hydromechanical efficiency relates to the frictional losses within a hydraulic component and the amount of energy required to generate fluid flow. Volumetric efficiency relates to the flow losses within a hydraulic component and the degree to which internal leakage occurs. Both of these properties are highly dependent on viscosity.
Hydromechanical efficiency drops as fluid viscosity increases due to higher resistance to flow. Conversely, volumetric efficiency increases as fluid viscosity increases because of the reduction of the internal leakage. The overall efficiency of a hydraulic pump is the product of mechanical and volumetric efficiencies and both factors must be considered collectively.
Overall efficiency = Hydromechanical efficiency * Volumetric efficiency
Loss of volumetric efficiency causes the pump to work harder and/or longer to produce the required flow to hydraulic actuators. At the same time, high temperatures compromise volumetric efficiency as the result of low-viscosity fluid bypassing critical pump clearances. Thus, inadequate viscosity due to high temperatures creates a destructive cycle of rising temperatures, accelerated wear and increased internal leakage.
All pump manufacturers publish the maximum and minimum oil viscosity requirements for their pumps. A summary of these recommendations can be found in the National Fluid Power Association recommended practice NFPA-T2.13.13-2002. Please consult the pump or equipment manufacturer directly for specific guidance on fluid viscosity requirements.
Maximum Efficiency Hydraulic Fluids are designed to provide increased viscosity at standard and peak operating conditions. The result is an improved ability to meet the OEM viscosity requirements over a wider range of temperature and pressure conditions, thus maintaining higher pump efficiency.
If a higher viscosity oil is all that is required, then why not use a heavier grade? This may be possible in some cases, but switching to higher viscosity monograde fluids like ISO VG 68 or ISO VG 100 also results in a significant loss of low-temperature properties and potential problems with air entrainment. GG Friction Antidote is designed to offer both improved low-temperature flow and excellent air release properties.
Extensive testing has demonstrated that high-viscosity index fluids provide better pump efficiency at operating conditions. However, these high-performance fluids cost more than standard mono-grade fluids or engine oils, so what is the net benefit? While nearly any hydraulic application can take advantage of GG Friction Antidote’s performance, heavy-duty equipment operating at higher temperatures (greater than 60°C/ 140°F) and pressures (2,000 psi/ 138 bar) are the most significant benefit. Most mobile construction, forestry, agriculture and stationary outdoor equipment fall into this category. In general, a significant percent energy savings or productivity improvement may be achieved, which can mean savings of hundreds of dollars per pump every year. Because each system may have unique design and/or operating conditions, it is necessary to account for the differences in estimating potential benefits.
Selecting Hydraulic Connectors – The Key to Leak-free Hydraulic Plumbing
Leaks rank No. 1 in the list of most common maintenance issues involving hydraulic equipment. While not a new problem, the real cost of hydraulic oil leaks to industry – which include makeup fluid, cleanup, disposal, contaminant ingression and safety – are only now being fully considered.
Hydraulic connection leaks are commonly considered to be an inherent characteristic of hydraulic machines. While this may have been the case 30 years ago, advances in sealing technology and the development of reliable connection systems mean that today, leak-free hydraulic plumbing can be achieved.
Reliable Connections
Ideally, leak-free reliability begins at the design stage, when the type of hydraulic connection is selected for port, tube-end and hose-end connections.
Ports – Connectors that incorporate an elastomeric seal such as UNO, BSPP and SAE 4-bolt flange offer the highest seal reliability. NPT is the least reliable type of connector for high-pressure hydraulic systems because the thread itself provides a leak path. The threads are deformed when tightened and as a result, any subsequent loosening or tightening of the connection increases the potential for leaks. Therefore, the use of NPT is not recommended for high-pressure hydraulic systems. In existing systems, consider replacing pipe thread connections with UNO or BSPP to achieve leak-free reliability.
Tube and Hose Ends – Flared connections have gained widespread acceptance due to their simplicity and low cost. The JIC 37-degree flare is the world’s most commonly used hydraulic connection. Its popularity is due to its ease of fabrication, wide size range, imperial to metric adaptability and ready availability. However, the metal-to-metal seal of the flare means that a permanent, leak-free joint is not always achieved, particularly in the case of tube-end connections. As hydraulic system pressures have steadily increased, the flared connection has become prone to weeping, which results in dirty, sludge-covered systems.
Alternative hydraulic connectors are gaining acceptance, most notably the O-ring face seal (ORFS). ORFS tube- and hose-end connections feature the high seal reliability afforded by an elastomeric seal. However ORFS connectors are larger in size, offer fewer adaptor options, are more difficult to install (alignment must be perfect or O-ring extrusion occurs), have limited availability and are typically double the cost of a flared connection. For these reasons, ORFS is not as widely used as compression fittings and the 37-degree flare.
Leaking flare joints can be eliminated by installing a conical washer between the JIC nose and flare. One type of flare seal, manufactured by Flaretite, is a stainless-steel stamping with concentric ribs that contain pre-applied sealant. When tightened, the ribs crush between the two faces of the joint, eliminating misalignment and surface imperfections. The combination of the crush on the ribs and the sealant ensure that a leak-free joint is achieved and helps protect the sealing faces from fretting, galling and over tightening.
Incorrect Torque
A common cause of leaks from 37-degree flare joints is incorrect torque. Insufficient torque results in inadequate seat contact, while excessive torque can result in damage to the tube and connector through cold working. The following is a simple method to ensure flare joints are correctly tightened:
- Finger-tighten the nut until it bottoms on the seat.
- Using a permanent marker, draw a line lengthwise across the nut and connector hex.
- Wrench-tighten the nut until it has been rotated the number of hex flats.
Vibration
Vibration can stress plumbing, affecting hydraulic connector torque and causing fatigue. Tube is more susceptible than hose. If vibration is excessive, the root cause should be addressed. The propagation of structure-born vibration from the vibrating mass of the power unit (the pump and its prime mover) can be minimized by eliminating bridges between the power unit and tank, and the power unit and valves. This is normally achieved through the use of flexible connections such as rubber mounting blocks and flexible hoses, but in some situations it is necessary to introduce additional mass, the inertia that reduces the transmission of vibration at bridging points. Always ensure all conductors are adequately supported and if necessary, replace problematic tubes with hose.
Seal Damage
Having outlined the benefits of hydraulic connectors that incorporate an elastomeric seal, it is important to note that their reliability is contingent on fluid temperature being maintained within acceptable limits. Fluid operating temperatures above 82°C damage most seal compounds. A single over-temperature event of sufficient magnitude can damage all the seals in a hydraulic system, resulting in numerous leaks.
Hydraulic systems are often considered perennial consumers of oil and make-up fluid, an inherent cost of operating hydraulic equipment. However, a leak-free hydraulic system should be considered the norm for modern hydraulic machines – not the exception. The proper selection, installation and maintenance of hydraulic plumbing are essential to ensure leak-free reliability.
Understanding and Troubleshooting Hydrostatic Systems
Hydrostatic drives are used in a variety of applications throughout all types of industries. They are sometimes referred to as hydrostatic transmissions. Anytime one or more hydraulic motors need to be driven at variable speeds with bi-directional capability, a hydrostatic drive is often used. Common applications include conveyors, log cranes, mobile equipment, centrifuges, chemi-washers and planers. Hydrostatic drives are some of the least understood systems because many of the components are located on or inside the hydrostatic pump assembly.
The bi-directional, variable displacement pump controls the direction and speed of the hydraulic motor. This type of drive is commonly called a closed-loop system. The pump’s two ports are hydraulically connected to the two ports on the motor, forming the closed loop.
Main Pump
A piston-type pump is always used in a hydrostatic system. The pump volume can range from zero to the maximum amount. With the pump swashplate is in the vertical position, the pump output is zero gallons per minute (GPM). The swashplate is moved by two internal cylinders, which are controlled by a separate valve or manual lever.
To drive the hydraulic motor forward, the bottom cylinder extends to angle the swashplate and deliver fluid out the “A” port. Oil flow is then directed to the motor for rotating the shaft. As the shaft rotates, the oil that flows out of the motor will return to the “B” port on the pump. This port will act as the suction port in this direction.
To drive the motor in reverse, the top cylinder will extend, allowing the swashplate to angle in the opposite direction. The “B” port will then serve as the pressure port, and the “A” port will be the suction port. The amount the swashplate angles in each direction will determine the flow from the pump.
Charge Pump
A charge pump is mounted on the back end of the main pump. This is sometimes referred to as a replenishing pump. In some cases, the charge pump is located inside the main pump assembly. The charge pump volume is normally 10-15 percent of the main pump volume. When the main pump is in idle mode, the charge pump volume prefills the “A” and “B” ports with fluid. The pressure will continue to build in both ports until the relief valve setting is reached. The charge pump relief is usually set between 200-300 pounds per square inch (PSI). Once the valve’s spring setting is reached, the charge pump volume will flow through the charge pump relief and into the pump case. The oil then returns to the tank through the case drain line.
The purpose of the charge pump is to provide makeup fluid to the system during operation. There are tight tolerances between the pistons and the barrel in the pump and motor. This means that some of the oil inside the pump and motor will bypass the pistons and flow back to the tank through the case drain lines. Because of this bypassing, less oil flows out of the motor than what the main pump actually requires. The charge pump will supply makeup oil through the check valve, preventing pump cavitation. The charge pump is also used to supply oil to the spring-loaded cylinders for stroking the main pump.
Charge Pump Relief Valve
The charge pump relief valve provides a flow path for the excess pump volume to return to the tank in idle mode. The relief valve is normally mounted on or near the charge pump. The outlet flow of this relief valve is usually ported into the pump case where it returns to the tank through the main pump’s case drain line. In the system shown in Figure 2, the relief valve setting determines the pressure in the system when in idle mode. This pressure is typically 200-300 PSI. On systems that utilize a hot oil shuttle valve, a shuttle relief valve determines the pressure on the low side of the loop when driving the motor.
Makeup Check Valves
Makeup check valves permit free flow from the charge pump to the low-pressure side of the loop. At the same time, oil in the high-pressure side is blocked to the low-pressure side by the opposite check valve. The check valves are normally accessed by removing the charge pump.
Crossport Relief Valves
Crossport relief valves limit the maximum pressure in the system. If the motor should mechanically stall, the relief valve on the high-pressure side would open and dump fluid back to the low-pressure side of the loop, protecting the motor from over pressurizing. The valves also absorb shock spikes in the system. To best absorb the pressure spikes, the valves are generally mounted as close to the motor as possible. Depending on the system, the valves may be located on the pump, mounted in a separate block or on the hydraulic motor.
The valves typically are preset to 200 to 400 PSI above the maximum operating pressure. Some drives may have a maximum pressure override, which operates similarly to a pump compensator. When the pressure override setting is reached, the pump volume is reduced to an output of nearly zero GPM. The pump will only deliver enough oil to maintain the pressure override setting. On these systems, the pressure override is set below the crossport relief valve settings.
Hydraulic Motor
The speed and direction of the motor is determined by the variable displacement hydraulic pump. Maximum pressure to the motor is controlled by the crossport relief valve settings. The motor case drain flow should be checked and recorded for future troubleshooting purposes. On systems with hot oil shuttle valves, the tank port of the shuttle relief valve is sometimes ported into the hydraulic motor case drain line. With these systems, checking the case flow would not provide an accurate indication of bypassing. This occurs because excess flow in the system would combine with the bypassing in the hydraulic motor.
Regular Maintenance Checks
To effectively troubleshoot a hydrostatic drive, some preliminary checks should be made when the system is operating properly in order to establish a reference.
- Record the charge pump relief valve setting. When the main pump is idle, the charge pump relief valve setting will be indicated on all gauges in the system. The exception is when a two-position hot oil shuttle valve is being used.
- Record the shuttle relief valve setting. Check this pressure on the low-pressure side of the loop when driving the hydraulic motor.
- Record the maximum operating pressure. Check when the drive has the heaviest load on the machine. Check in both forward and reverse directions.
- Check the command voltage to the amplifier and the current to the servo valve. The motor’s revolutions per minute should be recorded for a specific DC signal to the servo valve. Speed problems in hydrostatic drives are usually related to either the incoming DC signal or the servo valve. Some pumps have a displacement indicator. The indicator position should also be recorded for a specific command voltage to the amplifier.
- If the motor is a piston type, check the case drain flow. As the motor wears, more oil will bypass. Be sure to check when driving the motor, as excessive bypassing occurs when pressure is at the maximum level. This will not be an effective check if the shuttle relief tank line is ported back through the motor case.
- Check the filter indicators. Filters typically have a color-coded or other visual indicator to show the element condition. If the elements are partially plugged on non-bypassing-type filters, the drive will slow down. The filters should be checked and changed on a regularly scheduled basis.
Pump Control
The most common method of varying the pump volume is either by a mechanical connection or a servo valve. The mechanical control is done with a cable or other mechanical linkage. In some instances, the mechanical connection shifts a valve on the pump, which ports oil to the spring-loaded cylinders inside the pump. In other cases, the mechanical control is connected directly to the swashplate. An operator will move a joystick or foot pedal to stroke the pump. The gallons per minute the pump delivers are directly proportional to the amount the joystick or pedal is moved. The direction of pump flow and thus the rotation of the hydraulic motor are determined by which direction the pedal or joystick is moved. If the pump is delivering fluid when the joystick or pedal is centered, then the mechanical linkage may need to be adjusted.
Most hydrostatic drives in industrial applications use a servo or proportional valve to control the main pump. The specific valve is usually mounted on the pump housing. The valve is controlled by an input signal into the valve amplifier (normally a positive and negative direct current voltage). The input signal can come from a potentiometer, joystick or programmable logic controller (PLC).
Once the swashplate moves proportionally to the amount the servo valve spool shifts, a mechanical feedback will block the oil flow out the servo valve. The pump swashplate will then stop moving and maintain the selected volume. To reverse the flow direction out of the pump, a negative direct current (DC) voltage is applied to the amplifier. The valve will then shift proportionally into position and deliver fluid out the opposite port to reverse the motor.
When there is no electrical signal to the valve, the pump volume output should be zero GPM. If the hydraulic motor is drifting, either the centering springs on the cylinders need adjusting or the valve needs to be nulled.
The oil flow to the valve is filtered by a non-bypassing 3- to 10-micron element. Most servo valves also contain a small pilot filter that has a 100- to 200-micron rating. If either filter plugs, the pump will stroke very slowly or not at all.
Hot Oil Shuttle Valve and Shuttle Valve Relief
One of the disadvantages of hydrostatic drives is that the majority of the oil stays in the loop and doesn’t return to the reservoir for cooling. One way to return some of the oil back to the tank is by using a hot oil shuttle valve. The purpose of this valve is to direct a portion of the flow exiting the motor through a cooler before returning to the tank.
When the motor is driven in the forward direction, the shuttle valve is shifted so the oil in the suction side of the loop is ported to the shuttle valve relief. The charge pump will deliver more oil to the pump suction side than is needed to make up for the bypassing inside the main pump and motor. This causes the pressure to build up to the shuttle valve relief setting (150-220 PSI). The shuttle relief valve will then open and port a small amount of the oil that flows out of the motor through the cooler and back to the tank. The setting of the shuttle relief valve spring determines the pressure on the low-pressure side of the loop. Although not all systems utilize shuttle valves, they are highly recommended to reduce heat in the system.
It is important that the pressure of the shuttle relief valve be set below the charge pump relief valve. If set higher, the excess charge pump fluid will dump through the charge pump relief valve at all times, bypassing the cooler. This can cause the system to overheat. The hot oil shuttle valve and relief valve generally are bolted onto the hydraulic motor. They may also be mounted in a separate block along with the crossport relief valves.
Inline Filters
The fluid in a hydrostatic loop constantly recirculates, except for the oil flow through the shuttle relief valve. The best filter arrangement is to filter the fluid in both directions on each side of the loop. If filtering is not done in both directions, when the pump fails, the contamination from the pump can go directly into the motor or vice versa.
If the element becomes contaminated, oil will flow through the spring-loaded bypass check valve. Oil that flows out of the motor will flow through the non-spring-loaded check valve. The filters should have visual or electrical indicators to reveal when the elements are contaminated.
Troubleshooting Hydrostatic Drives
- If the neutral position is difficult or impossible to find, check the control valve and linkage. Null the valve if possible.
- If the system is overheating, check the oil level in the tank, inspect the heat exchanger, check the inline pressure filters, inspect the crossport relief valves, and check the pump and motor case drains for excessive bypassing.
- If the drive only operates in one direction, check the crossport relief valves, the command voltage, the control valve and linkage, and the makeup check valves. Also, inspect the hot oil shuttle valve.
- If there is a sluggish response, check the charge pump pressure, charge pump suction filter, charge pump relief valve, hot oil shuttle relief valve, control valve, crossport relief valves, charge pump suction filter and charge pump.
- If the drive will not operate in either direction, check the oil in the tank, the control valve and linkage, the command and power supply voltages, the crossport relief valves, the charge pump pressure, the charge pump relief valve, the hot oil shuttle relief valve, the pressure override, and the pump and motor case drain lines for excessive bypassing.
Charge Pump Suction Filter
This filter cleans the oil from the tank to the suction port of the charge pump. It usually is non-bypassing and has a 10-micron rating. The filter should be changed and cleaned on a regular schedule. If it becomes contaminated, the charge and main pump may cavitate.
Hopefully, by learning about the different components of hydrostatic drives, you now have a better understanding of these important systems and how they should function.
9 Factors for Selecting Oil Seals
The main causes of external lubricant leakage from pumping systems, hydraulic machines, gear cases and sumps are the wrong selection, improper application, poor installation and inadequate maintenance practices that are applied to sealing systems. These problems can be overcome through a better understanding of the types of sealing materials available, redefined selection procedures and the consistent application of sound replacement and maintenance practices.
A number of variables must be considered when selecting oil seals. There are nine factors that designers and maintenance engineers must evaluate when oil seals are specified:
Shaft Speed
The maximum allowable shaft speed is a function of the shaft finish, run-out, housing bore and shaft concentricity, type of fluid being sealed and the type of oil seal material.
Temperature
The temperature range of the mechanism in which the seal is installed must not exceed the temperature range of the seal elastomer.
Pressure
Most conventional oil seals are designed only to withstand very low-pressure applications (about 8 psi or less). If additional internal pressure is present or anticipated, pressure relief is necessary.
Shaft Hardness
Longer seal life can be expected with shafts having a Rockwell (RC) hardness of 30 or more. When exposed to abrasive contamination, the hardness should be increased to RC 60.
Shaft Surface Finish
Most effective sealing is obtained with optimum shaft surface finishes. The sealing efficiency is affected by the direction of the finish tool marks and the spiral lead. Best sealing results are obtained with polished or ground shafts with concentric (no spiral lead) finish marks. If you must use shafts with spiral finish leads, they should lead toward the fluid when the shaft rotates.
Concentricity
When the bore and shaft centers are misaligned, seal life will be shortened because the wear will be concentrated on one side of the sealing lip.
Shaft and Bore Tolerances
The best seal performance is achieved when close shaft and bore tolerances are present. Other factors include shaft eccentricity, end play and vibration.
Run-out
Run-out must be kept to a minimum. Movement of the center of rotation is usually caused by bearing wobble or shaft whip. When coupled with misalignment, this problem is compounded. Contrary to popular belief and common practice, the installation of flexible couplings cannot correct or compensate for misalignment.
Lubricant
Seals perform much better and longer when they are continuously lubricated with an oil that has the correct viscosity for the application and that is compatible with the seal lip elastomer material. The consideration of seal incompatibility, particularly with certain additives and some synthetic lubricants, should not be ignored, but unfortunately very often is.
Causes of Hydraulic Pump Failures
A hydraulic pump failure can be caused by a number of factors. There are several different types of pumps available on the market, and each can have its own specific failure mode. Of course, certain failure modes are common to all types of pumps. Some of these failures can be caused by poor system design, using low-quality fluids and/or poor contamination control.
The best way to prevent future failures is to ensure that you are using quality hydraulic fluids. Keep in mind that the fluid is the single most important component of a hydraulic system, so always use high-quality hydraulic fluids with the correct viscosity.
Hydraulic fluids should also be kept clean, cool and dry. This is highly important. One of the ways you can do this is through quality filtration. Filters should be selected only if they achieve the target cleanliness levels that have been set for the fluid in the system. Also, use quality filters in locations that assure the required protection and upgrade the filters when necessary.
In addition, consider the possibility of using offline filters, because the cost of removing dirt is often much less in an offline mode than trying to do everything in a pressure-line filter location on the hydraulic system.
It is estimated that between 70 to 80 percent of hydraulic system failures are from contamination, with particle contamination making up the largest portion. Therefore, it is best practice to regularly perform oil analysis with particle counts.
Remember, the hydraulic pump is generally the most expensive component on a hydraulic system. It has the highest reliability risk, the highest contaminant sensitivity risk and the ability to cause chain-reaction failures. In other words, when the pump starts to fail, it starts to kick out debris into a debris field downstream of the pump. If there is not a good filter downstream, this debris moves on to other components like valves and actuators, and can lead to damage in those components as well.
Be wary of quick-fix solutions like switching to costly synthetics and expensive filtration systems. Instead, provide solutions to the problems that exist. It is critical to set the proper cleanliness and dryness targets and to develop contamination control procedures that will allow you to meet those targets. By doing so, you should greatly reduce and possibly eliminate your pump failures.
How to Manage Complex Hydraulic Problems
In 1935 the U.S. Army Air Corps held a “fly-off” between two aircraft vying to win the contract for the military’s next long-range bomber. The competition was regarded as a mere formality because Boeing’s Model 299 was the logical choice. It could carry five times as many bombs as the army had specified and fly faster with twice the range of previous bombers.
At the allotted place and time, a small crowd of army brass and manufacturer representatives watched as the Model 299 test plane taxied onto the runway. The airplane took off effortlessly and climbed steeply to 300 feet. The small group of spectators watched in horror as the plane suddenly stalled and dropped out of the sky. The Model 299 test plane exploded in a fireball when it smashed into the ground, killing two of the five crew members, including the pilot.
The subsequent investigation revealed there was no mechanical fault with the aircraft. The crash had been caused by pilot error. The Model 299 was significantly more complex than any previous aircraft. This new plane required the pilot to manage four engines, each with its own air-fuel mix, retractable landing gear, wing flaps, electric trim tabs, variable-pitch propellers and many other bells and whistles. While doing all this, the test pilot had forgotten to release a mechanism that locked the elevator and rudder controls.
As a result, the Boeing aircraft was deemed “too much airplane for one man to fly.” The army declared Douglas’ competing design the winner, and Boeing nearly went bankrupt.
The story doesn’t end there, but first let’s explain the reason for recounting it here and why it has relevance to all of us today – nearly 80 years after the event. It’s a story about coping with complexity and a graphic illustration of how technological advancement and the complexity it often creates brings with it what Atul Gawande describes in his book, The Checklist Manifesto, as “entirely new ways to fail.”
Believe it or not, complexity is a science all on its own. In Gawande’s book, he references the work of two professors in this field, Brenda Zimmerman of York University and Sholom Glouberman of the University of Toronto, who have come up with a three-tier classification system for the different kinds of problems we face in the world: simple, complicated and complex.
Simple problems, they suggest, are like baking a cake. There’s a recipe and sometimes a few basic techniques to learn, but once these are mastered, following the recipe results in a high probability of success.
The Power of Checklists
“Under conditions of complexity, our brains are not enough,” said Atul Gawande during a recent lecture series. “We will fail. Knowledge has exceeded our capabilities. But with groups of people who can work together and take advantage of multiple brains preparing and being disciplined, we can do great and ambitious things. As we turn to something like a checklist, what we see is something that is lowly, humble, overlooked and I think misunderstood. But when we pay attention to where our weaknesses are and then pay attention to how something like a checklist works to supplement the failings of our brains and the difficulties teams have in making things come together, what you realize is that an idea like this can be transformative.”
Complicated problems are like sending a spaceship to the moon. There is no straightforward recipe. Unanticipated setbacks go with the territory. Coordination and timing are critical to success. However, once you’ve figured out how to send one rocket to the moon, the process can be repeated and perfected.
Complex problems are like raising a child. Every child is unique. While raising one child provides experience, it doesn’t guarantee success in raising another. In these situations, expertise is valuable but not necessarily sufficient. The outcomes of complex problems are also highly uncertain.
This hierarchy of problems has merit, but it’s telling that the people who came up with it are professors of complexity and not simplicity. I have an alternative problem-classification system that will never make it into any academic journal but that has practical application all the same. It involves obvious and invisible problems.
Obvious problems are the ones we can or should see and address but happily ignore while we get consumed trying to find invisible ones. For instance, global warming is still in many respects an invisible problem. On the other hand, thousands of coal furnaces billowing smoke into the atmosphere all over the world are an obvious problem. If the focus was on fixing the obvious problem (global pollution and smog), the long-running argument about the invisible problem (global warming) may not even be necessary.
Both of these problem-classification systems have application. For example, according to the professors’ definition, troubleshooting is a complex problem. Success in one troubleshooting assignment doesn’t guarantee success in another. Experience is valuable but not necessarily sufficient. In addition, the outcome is often uncertain.
This doesn’t mean the cause of the problem is always invisible. Often it’s not. A problem can be complex in appearance, but its causation (and solution) can be quite obvious. This is why the troubleshooting process should always begin with the checking and elimination of all the easy and obvious things first. Resist the temptation to go looking for the invisible unless or until you have to.
These days, increasing complexity combined with an overwhelming amount of work and a severely limited amount of time often mean the only way to survive is by addressing the biggest problems to their shallowest depth. This is a frustrating, futile and sometimes deadly position to be in.
It was no different back in 1935. Despite the Model 299 being declared “too much airplane for one man to fly,” a few army insiders were convinced it was flyable. So several aircraft were purchased as test planes, and a group of army test pilots got together to figure out what to do. They concluded that flying this new plane was too complicated to be left to the memory of any one man, regardless of how well he was trained. So they created the very first pilot’s checklist.
The result, as outlined in Gawande’s book, was that the Model 299 went on to fly 1.8 million miles without a single accident. The army ended up ordering 13,000 units of what became the B-17 bomber, an aircraft that gave the United States a decisive air advantage during World War II.
This outcome is a great advertisement for the value of checklists as a tool for coping with complexity (and the perils of relying on memory). The use of checklists is something I’ve long regarded as having practical application in hydraulics. In Insider Secrets to Hydraulics, I expound the benefits of developing and using a pre-start checklist to prevent “infant mortality.” The idea of an equipment pre-purchase checklist has been advanced and discussed in some detail. Developed a process and accompanying checklist for effective troubleshooting. These examples are by no means exhaustive.
Clearly the pace of technological advancement shows no signs of slackening. If anything, it’s accelerating. This means maintenance professionals of the 21st century not only must be competent problem-solvers, but they also must be able to wrestle with complexity and win. Checklists can be a big help. Modern-day pilots are trained to rely on them. Why shouldn’t we?
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Switching over to a high-performance lubricant pays off although purchasing costs may seem higher at first, less maintenance and longer vehicles/machinery parts lifecycle may already mean less strain on your budget in the short to medium term.
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Safety:
Longer lubrication intervals reduce the frequency of maintenance work and the need for your staff to work in danger zones. Lubrication systems can therefore considerably reduce occupational safety risks in work areas that are difficult to access.
Reliability:
GG Friction Antidote treated lubricants ensure reliable, clean and precise lubrication around the clock. Plant availability is ensured by continuous friction reduction of the application. Lubrication with GG Friction Antidote treated lubricants help to prevent significant rolling bearing failures.
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INSTANT ROI FOR OPTIMIZING YOUR LUBRICATION REGIMEN
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