Hydraulic cylinder parameters

This page contains basic information about hydraulic cylinders for those unfamiliar with the terminology and basic parameters that specify hydraulic cylinders.

Standardization
Generally hydraulic components, cylinders included, follow industry standards. Therefore, you don't generally need to replace a hydraulic cylinder on a piece of equipment with it's original part number. All you need to do is determine what combination standardized parameters you have and find a cylinder which matches.

Terminology
A helpful diagram of the construction of a hydraulic cylinder is located on page 5 of this document. It shows a cutaway view of a hydraulic cylinder and has numbered call-outs pointing to the different components.

Bore
The bore is how fat the cylinder is. It's the inner diameter of the cylinder housing which is also equal to the diameter of the piston inside of the cylinder (note the piston is not the piston rod which moves in and out of the cylinder).

The bore size determines how much force the cylinder generates. Bigger bore sizes generate more force, and also move more slowly. Smaller bore sizes are faster and weaker.

Generally the bore size is 0.5" less than the outer diameter of the cylinder.

Bore sizes are generally standardized numbers like 2", 2.5", 3", 3.5", and 4".

Stroke
The stroke is the amount of travel the piston rod can move in and out. It can be found by subtracting the maximum extended length of the hydraulic cylinder from it's minimum compressed length.

Generally the compressed length is given along with the stroke. To find the maximum length of the cylinder, you have to add the two together.

Stroke lengths are standardized more or less in 2" increments from 4" to 24", and larger increments above those lengths, such as 6".

Another standardization is that the compressed length for cylinders of any given stroke is usually the same between cylinders. For example, most cylinders with a 12" stroke will have a 22.25" compressed length.

Bore x Stroke (x Piston rod)
Typically hydraulic cylinders are broadly categorized in Bore x Stroke. For example, a 4"x24" cylinder has a 4" bore and a 24" stroke. If there is a third number, it's the diameter of the piston rod.

The bore x stoke will give you the basic mechanical characteristics of the cylinder: how far it moves and how much force it can produce.

Piston rod
The piston rod is the rod that comes out of the cylinder. Generally it's size is determined appropriately for you by the manufacturer, based on the other parameters of the cylinder.

PSI rating
The psi rating is generally 2500 or 3000 psi. The 3000 psi rated cylinders are more expensive, but might be necessary depending on the system.

End mounts
The end mounts determine what mechanical interface the cylinders mount to. There are two general types: clevis and cross bar. The clevis mount is the most common has two ears which can go on either side of a plate. They use a 1" pin diameter and generally have just over 1" of clearance from ear to ear. They are generally all standardized. Cross bar mounts are a hole or bushing for a rod to go through.

Ports
Please see Hydraulic Connectors for more information. Below is information most relevant to existing old equipment that may be encountered.

Ports can be very confusing because there are a number of different types. The most common are NPT ports which are just pipe threads. They are the same as pvc pipe threads at home depot, and you can buy them and see if they mate to your equipment as a test. However there are multiple types of NPT ports, in addition to all the other port types.

Here is a post explaining some of this :

Quote:

Just to clear up any confusion (and maybe cause some more [img]/forums/images/graemlins/blush.gif[/img]), your threads are NPTF, most likely 1/2". Here is a description of the different pipe threads.

NPTF This is a dryseal thread; the National pipe tapered thread for fuels. This is used for both male and female ends. Several trade associations no longer recommend this thread connection for use with hydraulics but it is still widely used and fittings are easy to obtain and connection combinations cover the entire spectrum of types.

The NPTF male will mate with the NPTF, NPSF, or NPSM female.

The NPTF male has tapered threads and a 30° inverted seat. The NPTF female has tapered threads and no seat. The seal takes place by deformation of the threads. The NPSM female has straight threads and a 30° inverted seat. The seal takes place on the 30° seat.

The NPTF connector is similar to, but not interchangeable with, the BSPT connector. The thread pitch is different in most sizes. Also, the thread angle is 60° instead of the 55° angle found on BSPT threads.

NPSF The National pipe straight thread for fuels. This is sometimes used for female ends and properly mates with the NPTF male end. However, the SAE recommends the NPTF thread in preference to the NPSF for female ends.

NPSM National pipe straight thread for mechanical joint. This is used on the female swivel nut of iron pipe swivel adapters. The leak-resistant joint is not made by the sealing fit of threads, but by a tapered seat in the coupling end.

Even though NPTF fittings are dryseal, almost everyone uses some type of thread sealant when used in hydraulic applications.

End quote

Here are more resources for identifying other types of threads.    

Accessories
You will need a swivel connector to connect your hydraulic hose to the cylinder, because the hose is rigid. For example you might want to pick up a 1/2" NPT male to 1/2" NPT female if using 1/2" hose with NPT connectors.

Places to buy
Try surplus center, baily hydraulics, northern tool, or dalton hydraulics.