How to control the speed of a hydraulic cylinder?

By Josh Cosford, Contributing Editor

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Hydraulic cylinders are unique in that they&#;re nearly 100% volumetrically efficient, although they are subjected to losses from mechanical efficiency. This means that nearly all the fluid stuffed into them results in proportional linear movement, but only some of the pressure required by them is to move their loads. Regardless, speed calculations with cylinders are reasonably accurate, making controlling velocity fairly straightforward. Here are 5 ways to control hydraulic cylinder speed.

1. Meter-in flow control &#; The most common method to control the speed of a hydraulic cylinder is to limit the rate of flow entering its work ports. You could install meter-in flow control valves directly at the work ports, along the work lines or back at the valve stack or manifold. It&#;s imperative that the flow control employs a reverse-flow check valve to allow free flow out of the work ports for reasons I&#;ll explain shortly.
2. Meter-out flow control &#; As you&#;d expect from the description, and contrary to the first example, metering out inhibits fluid from exiting the cylinder to control speed. The old pneumatic adage, &#;When in doubt, meter out,&#; should give you much doubt. In some circumstances, metering out can create pressure intensification, leading to possible leakage or failure. Never meter out when the rod-side fluid is tasked to hold the load of a double-acting cylinder, which, when exposed to retraction pressure, will combine with load pressure to raise rod-side pressure to danger levels. The possibility for intensification is why work-port needle valves are not popular for hydraulic cylinders, necessitating the reverse flow check valve.
3. Needle valves &#; As already mentioned, you need to be careful here. But in many circumstances, this is the most effective and cost-sensitive method to control cylinder speed. Without check valves, needle valves restrict flow in both directions, which is suitable for applications with low-speed gravity down.
4. Priority type 3-port flow control &#; This valve has a single inlet and dual outlet, and the adjustment controls the proportion of flow out of each of those ports. The priority (CF) port is often pressure compensated. It rules the flow hierarchy so that flow is no longer available at the third port (EF) if incoming flow drops below the priority flow setting.

   You often see all-in-one 3-port flow controls with built-in relief valves, making them perfect for controlling one or two actuators in the system. Be mindful in applications where inlet flow is high while controlled flow is low, as all incoming flow is subjected to the operating pressure, leaving any excess flow wasted as heat.

5. Proportional directional valves &#; A proportional valve varies its spool position using variable power by way of a PWM controller. The spool contains metering notches to provide a metered flow, which essentially operates like dual needle valves. However, energy loss can be mitigated depending on the spool type, pump type and control circuit.

If the valve spool moves a little bit, the valve flows a little bit. Pressure compensation, load sensing or sophisticated electronic control lend the proportional valve superior performance for controlling cylinder speed.

No matter the control method, providing your cylinders with a method to control speed is always helpful. Even a simple orifice will control hydraulic cylinder speed, although its performance could be improved compared to more advanced methods such as proportional control.

The speed of a hydraulic system is determined by the amount of flow delivered. Normally, flow controls are used to accomplish this. While many people are aware that a flow control or orifice will limit the hydraulic flow in a system, they may not realize that orifice size isn&#;t the only variable that will affect the flow and therefore the speed of a hydraulic actuator, such as a cylinder or hydraulic motor.

There actually are three variables that affect flow: the orifice size, the pressure difference between the inlet and outlet of the orifice, and the oil temperature.

Figure 1. The higher the upstream pressure reading
on gauge A (as compared to gauge B), the harder the
flow is pushed through the orifice.

Orifice Size

The size of the orifice is fairly straightforward. The bigger the hole, the more flow will pass through it. Many flow controls have a variable orifice size, so turning the adjustment counterclockwise will increase flow, while turning it clockwise will close the valve, limiting the flow and slowing down the actuator.

Pressure Difference

Whenever a flow control is adjusted so that it limits flow, there will always be a pressure drop across the orifice. Any restriction of flow causes back pressure to build upstream of the valve. The greater the pressure drop, the more flow will pass through it. Figure 1 provides a good example of this.

Figure 2. A fixed-orifice flow control symbol

Oil Temperature

You may notice some machines move more slowly at startup than they do once the oil gets up to temperature. This is to be expected, because the higher the oil temperature, the lower the oil viscosity will be. Thicker oil cannot move as rapidly through an orifice as thinner oil can.

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Figure 3. A variable-orifice flow control symbol

Fixed Orifice

The fixed-orifice flow control (Figure 2) may or may not be a proper component in a machine. By definition, it is nothing more than a hole of a specific size that cannot be adjusted. It may be simply a drop in line size. Sometimes when a 2-inch hydraulic line is teed off to a 1/4-inch line, a draftsman may represent this as a fixed orifice. If you remove the valves from a manifold, you will find large holes and small holes drilled through it. The fixed-orifice flow control symbol may represent one of the smaller holes.

It may also represent an orifice that can be removed from the manifold with an Allen wrench. Whatever form it takes, it should never be removed and replaced by a variable-orifice flow control. The designer had something specific in mind when calling for a fixed-orifice flow control. For whatever reason, the designer did not want it to be adjusted. Its purpose may be to synchronize flow to more than one actuator, or it may be for safety purposes to keep an actuator from moving too rapidly.

Figure 4. A flow control symbol with a bypass check valve

Variable Orifice

The variable-orifice flow control symbol in Figure 3 has a diagonal arrow to indicate that it can be adjusted. It often is called a needle valve because a common construction uses a conical needle that seats to close off the valve. The conical needle is called a vernier. The purpose of the vernier is to make the adjustment proportional to the number of turns made on the adjustment. A common number of turns between fully open and fully closed is five, so each full turn of the knob will change the orifice size by 20 percent.

Manual valves such as ball valves, gate valves and butterfly valves should never be used to control speed in a hydraulic system. These types of valves are meant to be open or closed. Keeping them partially open in a high-pressure hydraulic system will cause them to be unable to seat properly, and they will not close all the way. The vernier is specifically designed to adjust flow.

There are also cartridge-type flow controls that mount in a manifold or valve stack. While these are not true needle valves, they are engineered with a spool that is cut to limit flow.

It is not uncommon for either fixed- or variable-orifice flow controls to have a built-in bypass check valve (Figure 4). The flow control with a bypass will limit flow in one direction but will allow free flow in the opposite direction. The purpose of these controls is usually to enable independent forward and reverse speed control.

Figure 5. A pressure-compensating flow control symbol

Pressure Compensating

The pressure-compensating flow control (Figure 5) is designed to maintain a constant flow regardless of the pressure drop across it. This control is used in systems where the load weight changes, but it is important to maintain a constant speed. Without the &#;pressure-&#;compensating feature, a heavier load will move more slowly than a lighter load. The pressure-compensating flow control is available as either a fixed- or variable-orifice type.

Temperature Compensating

If the ambient temperature varies enough to affect production, the answer may be a temperature-compensating flow control. The additional symbol looks a bit like a thermometer. This control can be expensive, so it is unlikely to be found unless it is truly needed. The temperature-compensating flow control will maintain a constant flow regardless of any changes in oil viscosity. It is available as either a fixed- or variable-orifice type, and can also be a pressure-compensating control if the load weight changes constantly.

When I am called to help diagnose problems in a system and the issue concerns a flow control, the most common problem I find is that an incorrect type of flow control has been installed. It may be that the flow control specified by the designer failed to take certain conditions into account, or that a flow control has been replaced with one of the wrong types. If speed control is important to your operation, switching to one of these flow controls may help.

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