Fluid Power Engineering Solutions - FAQ

Frequently Asked Questions

Hydraulic Cylinders

How should I measure a cylinder?

When the cylinder is fully extended and fully retracted; measure the pin-to-pin length, measure the diameter of the rod, the bore of the cylinder will often be 1/2′′ – 6′′ smaller than the outer diameter of the barrel, and determine the type and size of ports. Lastly, determine the type and size of the end mounts.

What is a displacement cylinder?

A displacement (single acting) cylinder extends the rod by flowing pressurized oil behind it, which forces the rod out of the barrel like a piston. The cylinder rod retracts when the pressure is released. The diameter of the rod is the effective piston area.

How much force and speed will my cylinder have?

Force (lbs) is determined by multiplying the area of the piston by the pressure (PSI).

(π x r2 x PSI)

Speed (sec. per stroke) is determined by the volume of the cylinder (gal.) divided by the volume of the pump (GPM) x 60 sec per min.

( Vcyl ÷ GPM x 60)

Hydraulic Motors

What is the difference between a pump and a motor?

Pumps convert rotational forces on shafts into pressured oil to provide hydraulic components. For example, electric or gas motors turn pump shafts and pumps supply oil to cylinders. Most pumps have a large suction port and a smaller pressure port.

A hydraulic motor converts pressurized oil to one of its ports into rotational force on its shaft. Motors generally have equal size ports.

Pumps & motors sometimes look alike. A major difference is the shaft seal. The low-pressure suction chamber of a pump provides an ideal place to drain pressure away from the inside of the seal, so most pumps have very low-pressure shaft seals. Hydraulic motors usually require higher pressure shaft seals.

Hydraulic Accessories and Parts

How do I determine the size and type of fitting or port?

Measure the diameter of the threads, whether the fitting has a tapered thread (NPT) or a straight thread (SAE, JIC, ORB), and whether it is angled or swivels.

How do I measure a hose?

Measure the overall length of the hose, including the fittings. Determine the size and type of fittings. Usually, the outer diameter is 1/4″ to 3/8″ larger than the  nominal size.

How to measure a seal?

There are three common ways to measure a seal: the inner diameter (I.D. ), the outer diameter (O.D. ), and the height. It’s best to take these measurements from the cavity where the seal is located and the shaft that passes through it.

What is the best oil to use?

Make sure to use the oil recommended by the equipment manufacturer. Most hydraulic systems use AW32 (light viscosity) or AW46 (medium viscosity) hydraulic oil. Buying cheap hydraulic oil is not cheap when your system overheats and your pump fails!

Hydraulic Pumps

Hydraulic Pump Failures

We see many hydraulic pumps with a variety of problems. Among the most common problems are:

Long term wear: If a pump has thousands of hours of reliable work behind it, it may just wear out. Seals become hardened, critical surfaces become worn, and bearings start to fail. A gradual decrease in power and an increase in leakage will signify the onset of this condition. Pumps of this type can be repaired, but are usually  more economical to replace.

Contamination: This is a more common cause of wear, and can lead to catastrophic pump failures. Dust from the air entering and leaving the reservoir, rust particles from the inside of the tank, and wear particles from the pump and other system components are all sources of contamination. It is similar to long-term wear: gradual power loss due to metal scoring, burred gear teeth, and parts worn to excessive clearances. It is important to filter hydraulic oil by a fine (typically 10 microns) filter on the return circuit. The filter must also be changed on a regular basis. Tank breathers should also have an air filtering mechanism. Filter elements should usually be changed more often than oil.

Cavitation or Aeration: Cavitation is the “starvation” of incoming oil to the pump inlet. Pump wants to take in more oil than the tank can supply, either due to too-small or restricted suction hoses or because the shaft is turning too fast. Cavitation is caused by small inlet hoses, inlet strainers, too many fittings, and too much head lift. One common cause is too thick oil, especially when cold. 

The increased vacuum on the incoming oil causes dissolved gasses to become bubbles, similar to a bottle of soda when it is opened. The bubbles adhere to the metal surfaces in the pump and collapse violently when moved to the pressure side of the pump chamber. They gradually eat away the metal surfaces on the pressure side.

Aeration is a similar problem, the allowance of a stream of air bubbles into the inlet line through a small leak due to a hardened suction hose or loose or worn fitting. Sometimes small enough that they will not leak oil when the system is off. When the air bubbles move into the pressure side, they collapse and erode the metal the same as cavitation bubbles.

The symptoms will first be a whining noise, especially when the system is pressurized (loaded). It is sometimes mistaken for a bad bearing. The return oil may be foamy. In severe cases the tank may overflow with foamy oil. After the internal erosion is severe the pump will lose power.

Cavitated pumps may be rebuildable, but the system inlet problems must be fixed first.

*Let the oil warm up by running with no load and at a slower RPM if possible.

Overheating: This is an often-ignored cause of pump failure as well as other serious system problems. A hydraulic system produces heat and the oil absorbs it. The system also radiates heat, mostly from metal surfaces. The hotter it gets, the faster it radiates the heat so eventually the rate of radiation equals the rate of heat creation. If this temperature is over 75º C, bad things start to happen in the system. Seals get hardened and start to leak, hoses lose flexibility and crack, and the oil gets too thin to lubricate moving metal parts and they are scored or galled.

We recommend 60º C, measured on the tank, as a good maximum operating temperature.

The most usual means of ensuring adequate heat radiation is by using a good sized reservoir. And using a too-small reservoir is often responsible for a too-hot system. The “rule of thumb” is to size the tank for one minute’s oil flow from the pump, i.e. for a 16 GPM pump, use a minimum 16 gal. tank. (The rule for industrial systems is 3 minute’s worth of oil.) For systems with heavy constant loads, especially driving hydraulic motors, more cooling capacity will be required. For example, bush hogs and gravel shakers. Heat exchangers (radiators) are often installed when a large reservoir cannot be used. But they must have a power source to drive the fan, often impractical on smaller systems. Systems used for less than 15 minutes and then allowed to cool can use small reservoirs.

Pumps used in an overheated system usually don’t last long. First they leak as the seals break down, and then the insides are worn and galled from lack of lubrication. Remember, your system doesn’t care how difficult or inconvenient it is for you to control the temperature! If it gets too hot, you will pay.

What pump do I need for my application?

Pump selection depends on what flow & pressure will be required by the hydraulic system, i.e., how fast the cylinder or motor should run and how much force or torque it must develop. Then a pump can be selected which is capable of that flow & pressure. And then an engine or motor can be selected which is capable of driving that pump with sufficient power.

Selecting components for a hydraulic system can get complicated. We have a lot of experience replacing system components or building new ones. We’ll be happy to help you get the right one.

Hydraulic Systems

Do I have an open or closed center system?

On an open center system, oil can flow through the valve when it is in neutral. These systems normally use gear pumps or vane pumps. When the valve is in a neutral system, pressure is close to zero.

On a closed centre system, the oil flow will be blocked when the valve is neutral. These systems normally use piston pumps. When the valve is in neutral there is pressure in the system.

Heat Exchangers

Heat Exchangers (Radiators) This is the other common way of dealing with excess heat. Heat exchangers are very effective: a small heat exchanger will dissipate as much heat as a large reservoir, but they are expensive and require a fan, usually electric. It’s important to size the heat exchanger properly – we can help with that. Some industrial systems use oil-to-water heat exchangers, which are very effective and also compact. And they don’t produce hot air which may be a problem indoors. Of course, they require a source of cool running water, and a place to return it to.

How often should I change my oil and filter?

The filter should be changed when the pressure drop through it reaches 10 – 15 PSI. Lacking an indicator, the rule of thumb is every 6 months to a year, depending on use.

The oil should be changed if it starts to turn milky (due to water contamination), starts to break down (overheating, varnish deposits), or smells like it has burned. The oil in a hydraulic system is usually good for a long time, perhaps years, because it usually isn’t exposed to burning heat like oil in an engine. It’s much more important to change the filter. Sending an oil sample to your oil supplier or local lab is the best way to tell if your oil is contaminated or worn out.

FPES are specialists when it comes to filtration and can assist you in extending the lifecycle of your asset through proactive filtration.

Keeping it Cool – what size Reservoir to use?

How to prevent overheating The usual method is by using a large enough reservoir. That does 2 things: it gives the oil time to rest and give off its heat. And the larger metal surfaces radiate more heat.

How big a tank do I need? The rule of thumb is one minute’s pump flow as a bare minimum, i.e., if the pump flow is 16 GPM, the tank should hold no less than 16 gallons. More is better! Industrial systems often use 3 minute’s worth of oil. 

A heat exchanger is the other option. Systems which will only be run for a short time, 15 minutes or less, and then allowed to cool, can use much smaller tanks. Remember, your system doesn’t care if you have a good reason why you can’t install a larger tank! If it gets too hot, you will pay!

Overheating – how hot is too hot?

Overheating is one of the 2 most common causes of hydraulic system failure (the other is contamination).

How hot is too hot? If the reservoir is so hot you can’t hold your hand on it, it’s probably too hot. 65°C is as hot as we recommend for the reservoir. At that temperature one or more of your components is likely much hotter. Most seals start to lose elasticity at 75°C. Some components in the oil may start to break down. And the oil gets so thin at high temps that it doesn’t lubricate the moving metal parts sufficiently.

What makes it overheat? Friction in the pump, and internal leakage in pump, valve, or cylinder all create some heat. Also the power needed to push the oil through various holes and passages in valves, hoses & fittings makes it hotter. It often only takes 15 – 20 minutes for the heat to build up. Hydraulic motors running constantly usually create a lot of heat.

Where does the heat go? In most cases, the metal components dissipate heat. Oil that is not being used at the moment sits in the reservoir, where it has a chance to cool through the tank’s walls. The longer it rests in the tank, the more chance it has to cool off.

Where should an oil filter go?

Oil filters can be mounted on either the suction line or the pressure line. Installing on the return line (between the outlet of the valve and reservoir) allows you to use a finer filter, and does not restrict suction line flow. Suction lines are often fitted with strainers, however they are often forgotten, which results in sludge accumulating near them and blocking flow to the pump. Pumps with a blocked suction line don’t last long!

Speak with the experts at FPES to discuss your filtration requirements, we hold stock and have excellent relationships with key industry suppliers such as Parker and HYDAC.