hydraulic pump

Is it Time for a Pump Overhaul?

I want to go over how to determine the condition of the hardest working component of a hydraulic system – the pump.

As a pump wears in service, internal leakage increases and, therefore, the percentage of flow available to do useful work (volumetric efficiency) decreases.

If volumetric efficiency falls below a level considered acceptable for the application, the pump will need to be overhauled.

In a condition-based maintenance environment, the decision to change-out the pump is often based on remaining bearing life or deterioration in volumetric efficiency, whichever occurs first.

Volumetric efficiency is the percentage of theoretical pump flow available to do useful work. It is calculated by dividing the pump’s actual output in liters or gallons per minute by its theoretical output, expressed as a percentage. Actual output is determined using a flow-tester to load the pump and measure its flow rate.

Because internal leakage increases as operating pressure increases and fluid viscosity decreases, these variables should be stated when stating volumetric efficiency.

For example, a hydraulic pump with a theoretical output of 100 GPM, and an actual output of 94 GPM at 5000 PSI and 120 SUS is said to have a volumetric efficiency of 94% at 5000 PSI and 120 SUS.

When calculating the volumetric efficiency of a variable displacement pump, internal leakage must be expressed as a constant.

To understand why this is so, think of the various leakage paths within a hydraulic pump as fixed orifices. The rate of flow through an orifice is dependent on the diameter (and shape) of the orifice, the pressure drop across it and fluid viscosity. This means that if these variables remain constant, the rate of internal leakage remains constant, independent of the pump’s displacement.

Craig Cook

How to Replace Hydraulic Hoses

Heavy equipment uses a hydraulic pump, valve spools, and cylinders to perform their tasks. These components are interconnected with a series of steel tubes and steel reinforced rubber hoses, and the hydraulic oil may eventually begin to leak through these hoses, making it necessary to replace them. It can be a dirty job, but doing it yourself can save considerable costs and time.

Locate the problem hose. This may be obvious if the hose has burst, since they typically handle oil at over 2000 PSI pressure, and one that bursts will discharge a large amount of oil in a short time. If the situation is a small leak, though, you will need to observe where the oil is dripping, and follow the wet trail it leaves to the source. Never use your hands or body parts to find the leak. Use cardboard, paper or hydraulic leak detection fluid so no oil injection is accrued. A good hydraulic shop stocks leak detection additives that assist on location the leak safely.

Assess how many components must be removed to facilitate replacing the damaged hose. Always label the component removed by number and letter so replacement of parts can be re-installed with ease. This may include housings, guards, clamps, other hoses, hydraulic cylinders, and more. Follow the hose from one end to the other, noting the route you will use to uninstall and re-install it. Putting a number and letter on the ports and hose ends.

Determine if the hydraulic component the hose serves, or any other hydraulic components which must be removed have a live load, or weight on them. If the oil in the system you are disconnecting is under pressure, it may blow out forcefully when the fittings which hold it are loosened, causing oil to be discharged under pressure. Relieve the pressure from these cylinders or components before proceeding.

Make sure any attachments that are supported by the hydraulic cylinder that the hose operates are lowered to the ground or blocked or chained up. The weight of an attachment can crush a mechanic if it suddenly falls when pressure is relieved in the cylinder that is supporting it.

Get the tools you will need to perform the job of removing the hose. The fittings on the each end of the hose will be removed with a wrench, which may vary in size from 9/16 to over 1 1/2 inches. Many of these fittings are designed to swivel or turn as they operate, so two wrenches will be required to remove each of them. Hold the stationary side of the coupling with one wrench to prevent it from turning, and possibly damaging an O-ring while turning the other to separate the coupling.

Remove all clamps and attachments which will interfere with removing the hose. Often, the hydraulic cylinder itself will need to be removed or supported so that fittings can be accessed. Hydraulic cylinders are either bolted directly to the boss or fixture which it operates, or is anchored with a steel pin, such as the one in the illustration.

Loosen the fitting that attaches the hydraulic hose to the hydraulic system, either at a coupling, a cylinder, or the valve spool itself. Make sure the fitting turns at the threaded connection, and does not twist any other part. If needed, you may have to hold the fitting the hose is attached to with a separate wrench.

Pull the hose off of the equipment when both ends are unfastened. Be aware that some hydraulic oil may leak from either or both connections, and the oil that is remaining in the hose will also dump out, so having a bucket handy to catch this spillage is a good idea.

Plug the fittings that remain on the machine to keep debris from getting into the system during the interval the fitting is open. If you do not have oil dripping from the fitting, and do not have a plug with the correct threads on it, you may tie a clean rag around the fitting to protect it, but be careful if rain is in the forecast, since a rag won’t protect the system from being contaminated with water.

Wipe excess oil from the hose, and give us a call to get a new one weather we have it on the shelf or we have to make it for you. We supply replacement hoses and fittings which can be assembled while you wait, and are less expensive than ordering an original equipment manufacturer’s product.

Clean all the fittings on the equipment before re-installing the hose. Make sure there is no dirt in the tubing or fitting which will end up in the hydraulic system when you are finished.

Plug the ends of your new hoses with a special cap or a clean rag before routing it through the equipment to where it goes. This will keep dirt and debris from being forced into the hose while installing it. Remove these temporary plugs immediately before installing the fittings where they mate up with their counterparts.

Make sure the hose is in the correct location, and it has the proper amount of slack where needed when it is installed, then thread the fittings back on the cylinder or other component where they were removed. Tighten these connections snugly. You may have access to a specification book which recommends a torque to apply to each fitting, but failing this, tighten them as much as you can without risking damaging the seals or stripping the threads which hold them.

Replace any clamps, guards, or other components which were removed to accomplish the task. Align any cylinder pins which were removed, and re-install them, and return any split or snap rings which hold them in place.

Check the fluid level in the machine, crank it up, and check for leaks. If you have had a chance to clean all surfaces that were soiled by the initial leak, any leaks that occur now will be much easier to spot. Keep in mind some hydraulic circuits will require bleeding to remove air from the system prior to using the machine. This usually applied to steering and brake systems, but there are other situations where air can become trapped, such as a single action cylinder where the hydraulic supply is at a low point in the cylinder.

Craig Cook

Part 2 Of Hydraulic Troubleshooting Guide 101

Here’s part 2 of our 3 part series on hydraulic trouble shooting 101.

STEP 3 – Pump or Relief Valve…

If high pressure cannot be obtained in STEP 2 by running the pump against the relief valve, further testing must be conducted to see whether the fault lies in the pump or in the relief valve. Proceed as follows: If possible, disconnect the reservoir return line from the relief valve at point H. Attach a short length of hose to the relief valve outlet. Hold the open end of this hose over the reservoir filler opening so the rate of oil flow can be observed. Start the pump and run the relief valve adjustment up and down while observing the flow through the hose.

If the pump is bad, there will probably be a full stream of oil when the relief adjustment is backed off, but this flow will diminish or stop as the adjustment is increased. If a flowmeter is available, the flow can be measured and compared with the pump catalog rating. If a flowmeter is not available, the rate of flow on small pumps can be measured by discharging the hose into a bucket while timing with a watch.

For example if a volume of 10 gallons is collected in 15 seconds, the pumping rate is 40 GPM, etc. If the gauge pressure does not rise above a low value, say 100 PSI, and if the volume of flow does not substantially decrease as teh relief valve adjustment is tightened, the relief valve is probably at fault and should be cleaned or replaced as instructed in STEP 5.

If the oil substantially decreases as the relief valve adjustment is tightened, and if only a low or moderate pressure can be developed, this indicates trouble in the pump. Proceed to STEP 4.

STEP 4 – Pump…

If a full stream of oil is not obtained in STEP 3, or if the stream diminishes as the relief valve adjustment is tightened, the pump is probably at fault. Assuming that the suction strainer has already been cleaned and the inlet plumbing has been examined for air leaks, as in STEP 1, the oil is slipping across the pumping elements inside the pump. This can mean a worn-out pump, or too high an oil temperature.

High slippage in the pump will cause the pump to run considerably hotter than the oil reservoir temperature. In normal operation, with a good pump, the pump case will probably run about 20F above the reservoir temperature.

If greater than this, excess slippage, caused by wear, may be the cause. check also for slipping belts, sheared shaft pin or key, broken shaft, broken coupling, or loosened set screw.

I’ll give you step 5 and 6 in my next post.

Craig Cook

Part 1 Of Hydraulic Troubleshooting Guide 101

Many of the failures in a hydraulic system show similar symptoms: a gradual or sudden loss of high pressure, resulting in loss of power or speed in the cylinders. In fact, the cylinders may stall under light loads or may not move at all. Often the loss of power is accompanied by an increase in pump noise, especially as the pump tries to build up pressure.

Any major component (pump, relief valve, directional valve, or cylinder) could be at fault. In a sophisticated system, other components could also be at fault, but this would require the services of an experienced technician.

By following an organized step-by-step testing procedure in the order given here, the problem can be traced to a general area, and then if necessary, each component in that area can be tested or replaced.

STEP 1 – Pump Suction Strainer

Probably the trouble encountered most often is cavitation of the hydraulic pump inlet caused by restriction due to a dirt build-up on the suction strainer. This can happen on a new as well as an older system. It produces the symptoms described above: increased pump noise, loss of high pressure and/or speed. If the strainer is not located in the pump suction line it will be found immersed below the oil level in the reservoir (point A).

Some operators of hydraulic equipment never give the equipment any attention or maintenance until it fails. Under these conditions, sooner or later, the suction strainer will probably become sufficiently restricted to cause a breakdown of the whole system and damage to the pump. The suction strainer should be removed for inspection and should be cleaned before re-installation. Wire mesh strainers can best be cleaned with an air hose, blowing from inside out. They can slso be washed in a solvent which is compatible with the reservoir fluid. Kerosene may be used for strainers operating in petroleum base hydraulic oil. Do not use gasoline or other explosive or flammable solvents.

The strainer should be cleaned even though it may not appear to be dirty. Some clogging materials cannot be seen except by close inspection. If there are holes in the mesh or if there is mechanical damage, the strainer should be replaced. When reinstalling the strainer, inspect all joints for possible air leaks, particularly at union joints (points B, E, G, H, J, and K). There must be no air leaks in the suction line.

Check the reservoir oil level to be sure it covers the top of the strainer by at least 3″ at minimum oil level, with all cylinders extended. If it does not cover to this depth there is danger of a vortex forming which may allow air to enter the system when the pump is running.

STEP 2 – Pump and Relief Valve

If cleaning the pump suction strainer does not correct the trouble, isolate the pump and relief valve from the rest of the circuit by disconnecting at point E so that only the pump, relief valve, and pressure gauge remain in the pump circuit. Cap or plug both ends of the plumbing which was disconnected. The pump is now deadheaded into the relief valve.

Start the pump and watch for pressure build-up on the gauge while tightening the adjustment on the relief valve. If full pressure can be developed, obviously the pump and relief valve are operating correctly, and the trouble is to be found further down the line. If full pressure cannot be developed in this test, continue with step 3.

Step 3 and 4 will be included in next week’s post. Stay tuned.

Craig Cook

Back to the Basics: Hydraulic Pumps

I’m going back to the basics with a review of hydraulic pumps. Let’s dive right in!

Positive-displacement pumps, also called hydrostatic pumps, are used in fluid power motion control applications. They have a very small clearance between rotating and stationary parts. These pumps deliver a specific amount of fluid to the system for each revolution.

Positive-displacement pumps can be further divided into two categories: Fixed- and variable-displacement.

Fixed-delivery pumps provide a single, specific volume displacement per revolution.

In variable-displacement pumps, displacement per cycle can be varied from zero to maximum volumetric capacity. Some of the more widely used types of positive-displacement pumps are gear, piston, and vane types.

Gear pumps can be either internal or external styles. External gear pumps are one of the most popular types used in modern hydraulic systems. Gear pumps produce flow by using the teeth of two meshing gears to move the fluid. Their simple construction ensures limited purchase costs and servicing. Gear pumps work under heavy operating conditions and transmit high amounts of hydraulic power. They feature decent hydraulic, mechanical and volumetric efficiency, compact dimensions, and low weight/power ratio. This balance of efficiency and economy make external gear pumps a popular choice for auxiliary systems in a number of different machines.

External gear pumps can be equipped with straight spur (the most common type), helical, or herringbone gears. In operation, the drive gear and driven gear rotate, creating a partial vacuum at the pump inlet (where gear teeth unmesh) that draws fluid into gear teeth. Gear teeth mesh at the outlet, forcing fluid out of the pump.

Internal gear pumps contain one internal and one external gear. They pump fluid in the same manner as external spur gear pumps. In the basic design, the internal gear, which drives the outer gear, has one tooth less than the outer gear. As they mesh, the teeth create sliding seal points. Because their transition zone from low to high pressure (the area over the crescent) is relatively long, internal gear pumps can offer lower noise levels than some other types of pumps.

Gears are made of special steel and are often case hardened and quench hardened. Then gears are ground and fine finished. Proper tooth profile design and geometric proportions can reduce pulsation and noise levels during pump operation.

Piston pumps supply high flows at high rpm. Two types of piston pumps, axial- and radial-piston, are manufactured in both fixed- and variable-displacement versions. Axial-piston pumps contain one or more pistons that convert rotary shaft motion into axial reciprocating motion. An angled cam (or wobble plate) rotates, causing pistons to reciprocate and take fluid in as they move toward the thin part of the plate. Fluid is expelled as pistons approach the thick end. In one version, the bent-axis design, both pistons and shaft rotate, making a wobble plate unnecessary. Bent-axis pumps use the drive shaft to rotate pistons.

With the longer sealing paths along the piston walls, piston pump efficiencies tend to be higher than other types of pumps. In addition, variable-displacement pumps can provide savings by only providing the pumping necessary for the function, saving additional energy and costs.

Radial-piston pumps (fixed-displacement) are used especially for high pressure and relatively small flows. Pressures of up to 5000 psi are common. Variable-displacement is not possible, but sometimes the pump is designed in such a way that the plungers can be switched off one by one, so that a sort of variable-displacement pump is obtained.

Radial-piston pumps are characterized by a radial piston arrangement within a cylinder block. As pistons reciprocate, they convert rotary shaft motion into radial motion. One version has cylindrical pistons, while another uses ball-shaped pistons. Another classification refers to porting: check-valve radial-piston pumps use a rotating cam to reciprocate pistons; pintle-valve pumps have a rotating cylinder block, and piston heads contact an eccentric stationary reaction ring.

Rotary vane pumps (fixed and simple adjustable displacement) generally have higher efficiencies and lower noise levels than gear pumps. They can be used for mid pressures of 2500 psi and modern units can exceed 4500 psi in continuous operation.

Some types of vane pumps can change the center of the vane body, so that a simple adjustable pump is obtained. These adjustable vane pumps are in general constant pressure or constant power pumps: the displacement is increased until the required pressure or power is reached and subsequently the displacement or swept volume is decreased until equilibrium is reached.

A critical element in vane pump design is how the vanes are pushed into contact with the pump housing, and how the vane tips are machined at this very point. Several type of “lip” designs are used, and the main objective is to provide a tight seal between the inside of the housing and the vane, and at the same time to minimize wear and metal-to-metal contact. Forcing the vane out of the rotating center and towards the pump housing is accomplished using spring-loaded vanes, or more traditionally, vanes loaded hydrodynamically (via the pressurized system fluid).

Considerations when selecting a hydraulic pump

Pressure
Angle
Related torque
Weight
Mounting configuration

Craig Cook

Could have avoided 4 hours of downtime…

Check out this recent troubleshooting situation by one of our guys:

The machine in question had a complex hydraulic system, the heart of which comprised two engines driving ten hydraulic pumps. Six of the pumps were variable displacement and four of these had electronic horsepower control.

The symptoms of the problem were slow cycle times in combination with lug-down of the engines (loss of engine rpm). The machine had just been fitted with a new set of pumps.

The diagnosis of the mechanic in charge was that the hydraulic system was tuned above the power curve of the engines, that is the hydraulics were demanding more power than the engines could produce, resulting in lug-down and therefore, slow cycle times.

The other possible explanation of course, was that the engines were not producing their rated horsepower.

Due to the complexity of the hydraulic system, he knew that it would take around four hours to run a complete system check and tune-up. So in order to eliminate the easy things first, when he arrived on site he inquired about the condition of the engines and their service history.

The mechanic in charge not only assured him that the engines were in top shape, he was adamant that this was a “hydraulic” problem.

Four hours later, after running a complete check of the hydraulic system without finding anything significant, he was not totally surprised that the problem remained unchanged.

After a lengthy discussion, he managed to convince the mechanic to change the fuel filters and air cleaner elements on both engines.

This fixed the problem. It turned out that a bad batch of fuel had caused premature clogging of the engine fuel filters, which were preventing the engines from developing their rated horsepower.

Had the relatively simple task of changing the engine fuel filters been carried out when the problem was first noticed, an expensive service call and four hours of downtime could have been avoided.

ALWAYS check and eliminate the easy things FIRST.

Craig Cook

Noisy Hydraulics

Noise level in the workplace is always an issue. The high power density and corresponding high noise emission of hydraulic components means that industrial hydraulic systems are often the target of efforts to reduce noise levels in the workplace.

The dominant source of noise in hydraulic systems is the pump. The hydraulic pump transmits structure-borne and fluid-borne noise into the system and radiates air-borne noise.

All positive-displacement hydraulic pumps have a specific number of pumping chambers, which operate in a continuous cycle of opening to be filled (inlet), closing to prevent back flow, opening to expel contents (outlet), and closing to prevent back flow.

These separate but superimposed flows result in a pulsating delivery, which causes a corresponding sequence of pressure pulsations. These pulsations create fluid-borne noise, which causes all downstream components to vibrate.

The pump also creates structure-borne noise by exciting vibration in any component with which it is mechanically linked, e.g. tank lid.

The transfer of fluid and structure induced vibration to the adjacent air mass resulting in air-borne noise.

Reducing fluid-borne noise

While fluid-borne noise attributable to pressure pulsation can be minimized through hydraulicpump design, it cannot be completely eliminated. In large hydraulic systems or noise-sensitive applications, the propagation of fluid-borne noise can be reduced by the installation of a silencer.

The simplest type of silencer used in hydraulic applications is the reflection silencer, which eliminates sound waves by superimposing a second sound wave of the same amplitude and frequency at a 180-degree phase angle to the first.

Reducing structure-borne noise

The propagation of structure-borne noise created by the vibrating mass of the power unit (thehydraulic pump and its prime mover) can be minimized through the elimination of sound bridges between the power unit and tank, and the power unit and valves.

This is normally achieved through the use of flexible connections i.e. rubber mounting blocks and flexible hoses, but in some situations it is necessary to introduce additional mass, the inertia of which reduces the transmission of vibration at bridging points.

Reducing air-borne noise

The magnitude of noise radiation from an object is proportional to its area and inversely proportional to its mass. Reducing an object’s surface area or increasing its mass can therefore reduce its noise radiation.

For example, constructing the hydraulic reservoir from thicker plate (increased mass) will reduce its noise radiation.

The magnitude of air-borne noise radiated directly from the hydraulic pump can be reduced by mounting the pump inside the tank. For full effectiveness, there must be a clearance of 0.5 meter between the pump and the sides of tank, and the mounting arrangement must incorporate decoupling between the power unit and tank to insulate against structure-borne noise.

The obvious disadvantage of mounting the hydraulic pump inside the tank is that it restricts access for maintenance and adjustment.

If hydraulic system noise remains outside the required level after all of the above noise propagation countermeasures have been exhausted, encapsulation or screening must be considered.

Craig Cook

Is it Time for a Pump Overhaul?

How to determine the condition of the hardest working component of a hydraulic system – the pump.

As a pump wears in service, internal leakage increases and therefore the percentage of flow available to do useful work (volumetric efficiency) decreases.

If volumetric efficiency falls below a level considered acceptable for the application, the pump will need to be overhauled.

In a condition-based maintenance environment, the decision to change out the pump is often based on remaining bearing life or deterioration in volumetric efficiency, whichever occurs first.

Because internal leakage increases as operating pressure increases and fluid viscosity decreases, these variables should be stated when finding volumetric efficiency.

For example, a hydraulic pump with a theoretical output of 100 GPM, and an actual output of 94 GPM at 5000 PSI and 120 SUS is said to have a volumetric efficiency of 94% at 5000 PSI and 120 SUS.

When calculating the volumetric efficiency of a variable displacement pump, internal leakage must be expressed as a constant.

To understand why this is so, think of the various leakage paths within a hydraulic pump as fixed orifices. The rate of flow through an orifice is dependent on the diameter (and shape) of the orifice, the pressure drop across it, and fluid viscosity. This means that if these variables remain constant, the rate of internal leakage remains constant, independent of the pump’s displacement.

Craig Cook

“The Beast” We Use to Test All Repairs

Check out “The Beast”

Okay so it isn’t exactly a beast BUT it is a huge bench that allows us to test every aspect of any hydraulic component.

When you have us repair or rebuild any hydraulic component, it goes through rigorous testing on our hydraulic test stand.

We use the test stand for:

Pumps
Cylinders
Valves
Motors (spin test)

Craig Cook

Hydraulic Troubleshooting Basics

Here is a hypothetical hydraulic problem and how to trouble shoot it.

“We have a hydraulic system that operates two cylinders. The pump (piston-type) has failed – for reasons unknown at this time. The tank, valves and cylinders were cleaned and a replacement pump installed. The new pump is delivering a maximum pressure of 1,000 PSI and appears to be creating heat.”

In any troubleshooting situation, no matter how simple or complex the hydraulic system, always start with the basics. This ensures that the obvious is never overlooked. In order for the ‘obvious’ to be obvious, the fundamental laws of hydraulics must be kept in mind:

Hydraulic pumps create flow – not pressure.
Resistance to flow creates pressure.
Flow determines actuator speed.
Pressure determines actuator force.
Fluid under pressure takes the path of least resistance.
When fluid moves from an area of high pressure to an area of low pressure (pressure drop) without performing useful work, heat is generated.

Theory is great, but it always makes more sense when put into practice. So let’s apply these fundamentals to the above situation in a way that ensures the obvious things are not overlooked.

“The new pump is delivering a maximum pressure of 1,000 PSI…”

We know that a hydraulic pump can only produce flow (pressure is created by resistance to flow). It follows that if the pump can’t get oil it can’t produce flow. So, check that the reservoir is filled to the correct level, the breather is not clogged, the suction strainer or filter (if fitted) is not clogged, the pump intake isolation valve is fully open, and the pump intake line is otherwise unrestricted.

If the pump is producing flow, then an absence of pressure indicates an absence of resistance to flow. Knowing this, and that fluid under pressure always takes the path of least resistance, the task now is to find the point at which pump flow is escaping from the circuit. If you’re skilled in reading and interpreting hydraulic symbols, the system’s schematic diagram (if available) can be useful in identifying possible locations.

“The new pump… appears to be creating heat.”

Because heat is generated when there is a pressure drop, using an infrared thermometer to check the temperature of individual components will quickly lead us to the hottest part of the system – and the probable location of the internal leakage. Note that in a properly functioning system fitted with a piston pump, it is not unusual for the pump case to be the hottest part of the circuit.

The above checks should have taken less than 10 minutes. If nothing conclusive was revealed, I would continue the process of elimination using a flow-tester to conduct a direct pump test.

Craig Cook