Back To The Basics Part VIII — Why Cylinders And Actuators Aren't As Simple As They Seem

Diesel Progress North American Edition, July, 2001 by Russell Henke

The trouble with talking about hydraulic cylinders and actuators is that they appear to be so simple -- just a hunk of pipe with a rod sticking out of it to move a load back and forth. In reality, they are much more sophisticated and their successful application requires careful analysis and considerable expertise. Working with them in the field is no less demanding, where a mistake can have a catastrophic result.

Linear motors -- cylinders, actuators, rams, are all terms frequently used interchangeably -- must be selected on the basis of function they are to perform in the circuit and manner in which they will be installed.

The most obvious selection criteria for a linear motor is that it should be able to provide enough output force to do the job at hand. The most frequently used method is to divide the load (force) on the piston rod by available pressure to arrive at the piston area needed:

[A.sub.p] = Load (force)/Pressure

This is applicable whether dealing with an air or hydraulic system. Unfortunately, this doesn't tell the whole story. The steady state load may not actually be the maximum load. While the load is being accelerated, additional inertia forces are imposed. These follow Newton's second law, [F.sub.j] = ma: where [F.sub.i] is the inertia force; m is the mass being moved = w/g (where W = weight and g = 32/2 ft./sec/sec); and a = acceleration in ft./sec/sec.

In addition, if the load is being accelerated from zero velocity, there is a static friction force to overcome. As soon as the piston rod starts to move, the friction force decreases to dynamic friction conditions. The load conditions a cylinder is likely to encounter are depicted in Fig. 1. Note that the total force on the piston rod is the sum of the steady state load resistance, the inertia force during acceleration and the friction forces. There are two distinct periods: during start-up from standstill, where maximum force (breakaway) is encountered; and the steady state condition after the piston rod reaches constant speed. In the latter case there are no inertia forces, so the only loading on the piston rod is load resistance and friction. Load resistance and inertia forces may be calculated as previously indicated. Friction forces can only be approximated. A value of either dynamic or static coefficient of friction must be looked up in a handbook. These coefficients will correspond to the types of mate rials being used.

Friction force is then calculated thusly:

[F.sub.fs] = [[micro].sub.s] x N & [F.sub.fd] - [[micro].sub.d] x N

Where [F.sub.fs] = static friction force

[F.sub.fd] = dynamic friction force

N = force normal to direction of motion

[[micro].sub.s] = static coefficient of friction

[[micro].sub.d] = dynamic coefficient

Frequently load reaction on the piston is not the governing factor in selecting the size cylinder to be used in a given application. Structural strength of the piston rod may govern bore size. When a piston rod is extended against a load (Fig. 2), it is functioning as a column, that is, structurally. The greater the stroke, the worse this column loading factor becomes. For every rod, there is some limiting strength of stroke beyond which it will buckle under some critical load conditions.

These are the most widely used output or actuator devices. Fig. 3 illustrates typical construction and nomenclature. Hydraulic cylinders are usually said to be single acting if they can be pressurized or can move a load only in one direction. This type will have only one line connecting it to the system. The cylinder must be retracted by the load itself or an auxiliary means, such as a spring, counterweight, etc.

A cylinder is double acting when it can be powered in both directions -- extend and retract. Thus it can control a load both ways.

By definition, a cylinder is a linear motor in which the piston rod is equal to, or less than, one--half the cross section area of the barrel or tube. If the rod is greater than this, it is called a ram and is usually single acting.

A linear motor (Fig. 4) is said to be a single-stage cylinder, because there is only one movable member. A telescopic cylinder is one in which there are two or more stages, one resting within the other. These are most applicable where a long stroke is required and only a short space is available for the actuator in retracted position. For example, it is practical to build telescopic cylinders 10 to 15 ft. long when extended and only 4 to 5 ft. retracted.

Output devices, or actuators, can be divided into two basic classes: linear and rotary A typical linear hydraulic actuator is the cylinder, which converts the energy transferred to the system fluid by the pump to a mechanical output -- namely, the linear thrust of the piston rod. Rotary hydraulic actuators are commonly known as hydraulic motors and are available in two basic types: continuous-rotation and limited-rotation.

On the surface, cylinders appear to be relatively simple fluid power components; yet their proper application requires careful consideration and considerable expertise. The selection criteria for cylinders include: