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.
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.
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
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.
Figure 3. A variable-orifice flow control symbol
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
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
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.
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.