Measure the pin to pin length when the cylinder is fully extended and fully retracted, measure the diameter of the rod, the bore of the cylinder will typically measure 1/4″ – 1/2″ less than the outer diameter of the barrel, also determine the type and size of the ports. Finally, determine the type and size of the end mounts.

In a displacement (single acting) cylinder the rod is extended by flowing pressurized oil behind the rod, which forces the rod out of the barrel like a piston. Once the pressure is released, the load on the cylinder retracts the rod. The diameter of the rod is the effective piston area.

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)

You can download Don’s “Cheat Sheet” here. It calculates force, speed and many other things for you on an Excel spreadsheet.

See “How do I measure a cylinder?” Once you have that information click the above link for cylinders and choose one that matches your specifications.

A single acting cylinder only provides force one way (most snow plow and telescopic cylinders are single acting). A double acting cylinder provides force when extending and retracting.

A pump converts rotational force on its shaft into pressurized oil to supply hydraulic components. Example: electric or gas motor turns pump shaft, pump supplies oil to a cylinder. Pumps generally 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.

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

Measure the overall length of the hose, including fittings. Also, determine the type and size of fittings. The outer diameter is usually 1/4″ – 3/8″ larger than the nominal size.

The inner diameter (I.D.), outer diameter (O.D.), and height are the most common ways to measure a seal. It’s best to take these measurements from the cavity where the seal is located and the shaft which goes through it.

Use the oil recommended by the equipment manufacturer. Most systems use AW32 (light viscosity) or AW46 (medium viscosity) hydraulic oil. Cheap hydraulic oil is not cheap when your system overheats and your pump is ruined!

We see lots of hydraulic pumps with lots of different problems. But there are several problems which are most common:

Long term wear: If a pump has thousands of hours of reliable work behind it, it may just wear out. The seals get hard, critical surfaces get worn down, bearings wear out. The symptoms will be gradually decreasing power, & increasing leakage. Such pumps may be rebuildable, but often are more economical to replace. And they probably don’t owe you anything!

Contamination: This is a more common cause of wear, and sometimes catastrophic pump failure. Abrasive dust from the air breathing in & out of the reservoir, rust particles from the inside of the tank when it sits unused, and wear particles from the pump and other components of the system are all sources of contamination. The symptoms are similar to long term wear: gradually decreasing power due to scored metal surfaces, burred gear teeth, parts worn until the clearances are too great. Hydraulic oil needs to be filtered by a fine (usually 10 micron) filter on the return side of the circuit. And the filter must be changed regularly. Tank breathers should also have some air filtering mechanism. Changing the filter element is usually more important than changing the oil.

Cavitation or Aeration: Cavitation is the “starvation” of incoming oil to the pump inlet. The pump wants to pull in more oil than the line from the tank will allow, due to a too-small or restricted suction hose, or to turning the shaft too fast. Small inlet hoses, inlet strainers, too many fittings, too much head lift are the common causes of cavitation. Oil which is too thick, especially when cold, is another common cause*. 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 180º F, 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 150º F, measured on the tank, as a good maximum operating temperature.

The most usual means of insuring 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.

A pump converts rotational force on its shaft into pressurized oil to supply hydraulic components. Example: electric or gas motor turns pump shaft, pump supplies oil to a cylinder. Pumps generally 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.

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.

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 neutral system pressure is close to zero.

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

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.

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 is the best way to tell if your oil is contaminated or worn out.

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 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. 150°F 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 180°F. 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 valve, 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? The heat is radiated away mostly by the metal components. While the oil not being used at the moment sits in the reservoir, it has a chance to cool through the walls of the tank. The longer it rests in the tank, the more chance it has to cool off.

Use the oil recommended by the equipment manufacturer. Most systems use AW32 (light viscosity) or AW46 (medium viscosity) hydraulic oil. Cheap hydraulic oil is not cheap when your system overheats and your pump is ruined!

An oil filter can be mounted on the suction line or pressure line. We recommend mounting on the return line (between valve outlet and reservoir) so you do not restrict your suction line, and so a finer filter can be used. Strainers are often used in the suction line, but they are sometimes forgotten and when sludge builds up around them they block flow to the pump. A pump with a blocked suction line doesn’t last long!

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 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 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. 150°F 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 180°F. 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 valve, 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? The heat is radiated away mostly by the metal components. While the oil not being used at the moment sits in the reservoir, it has a chance to cool through the walls of the tank. The longer it rests in the tank, the more chance it has to cool off.

Brandon, Good question! I had to think about this one. But the answer is simple: you can just put tees in your 2 cylinder lines, and run the lines from both valves into them. You put a standard valve on each side. The first valve should have a power beyond adapter – run a line from it to the 2nd valve inlet, as shown in my sketch here.

When the valves are centered, they block the cylinder ports, so there won’t be any interference when the opposite valve is used.
You may want to put restrictors in the cylinder lines to keep it from going too fast for the lift.

NOTE: the tees could also be installed in the ports of one valve, with lines connecting to the other valve, so you’d only have 2 lines running out to the cylinder.