How do you specify a hydraulic cylinder?

A hydraulic cylinder is one of the simplest devices you can use to create force this side of hiring a guy (or thirty) to push on stuff. It’s as simple as having something at point A and then moving it to point B. Although I keep using the word simple, there is a little bit you need to know to align result with intention.

When you specify a cylinder, the most critical datum required is the amount of mass you wish to move, or you wish the cylinder to be capable of moving. The latter bit of information may be important to consider if you don’t want your cylinder to be borderline in its ability to move your mass; should a dust mote land on your load and stall it mid-cycle, a bit of safety factor should have been added to overcome such a variable.

When you know your mass, you must then consider the effect that mass has on the force required to move it. For example, a 1-ton load being pushed straight up will require just over 1 ton of force (2000 lb). However, a 1-ton load being pushed across the ground will require only enough force to overcome friction and acceleration, which could be very little if it’s on ice or quite a lot if it’s on tarmac. Regardless, the force of the cylinder should always be high enough for margin of error.

Once you know the nature of the mass being moved, you need to also consider the geometry involved in moving it. For a machine such as a hydraulic press, which moves typically up and down, the geometry is simple and requires no extra math. However, when the load being moved is not centered on the point of effort and at perpendicular angles to that point of effort, the force required by the cylinder changes.

If you have a crane, for example, the cylinder pushes on the boom often very far from the load. In fact, the load distance can be ten times the effort distance, sometimes more. This effect is like holding your fishing rod closer to the bottom; at some point, even the lightest carbon-fiber rod will feel heavy. So the closer your effort point is to the fulcrum, the more force is required by the cylinder to lift the load.

Another concern with the point of effort is the direction the cylinder acts upon the boom. If a cylinder is attached halfway between the load and the fulcrum, and if the cylinder acts at 45° to the boom, not all of the force applied by the cylinder is used to push the load upwards. In this case, half the force created is trying to push the boom outward instead of upward.

Boom force requirements are fairly complex and require the use of your high-school trigonometry. Like most of the math so far, there is not enough room to calculate examples in this short blog post. If you don’t know the math for these calculations, find someone who does, because a boom cylinder needs much more force output than the load being moved would suggest.

There is one critical formula I must discuss in length, however, which is the most important one required when specifying a cylinder. Once you arrive at the force required by your cylinder, a popular and simple formula helps you choose the bore size needed to achieve that force.

F = PA

Where F = force, P = pressure, A = area of the cylinder piston

So if we have a 10,000 lb force requirement, and we are limited to 4000 psi in our system, we must solve for area (in²) as follows:

A = F/P → in² = 10000 lb / 4000 psi → in² = 2.5

We need a cylinder piston with an area of 2.5 in², which requires us to use a 2-in. bore cylinder with a piston area of 3.14 in². When specifying a cylinder bore size, you will always need to round up to the next larger bore size to provide the safety factor I discussed earlier. Selecting cylinder bore size is often the first step in hydraulic machine design, with stroke length, speed and flow calculations coming next, which I will discuss my next blog.