By Josh Cosford, Contributing Editor
During everyone’s basic training in fluid power, we talk about “spring-centered” this and “spring-retract” that. Springs are used in nearly every component in hydraulics; valves, pumps and cylinders. So prolific in hydraulic components that it’s clear we’re taking springs for granted.
Ground compression springs are used in most pressure valves to force the spool or poppet closed until pushed open by hydraulic pressure. Relief valves, reducing valves, sequence valves and counterbalance valves all use one or more springs to achieve their function.
Directional control valves, with few exceptions, use a spring to either center or offset the spool or poppet. A solenoid coil creates a magnetic field that will push or pull against the spring-loaded pin to shift the valve. Removing power subsequently allows the spring to return the valve to its neutral position.
Pressure-compensated piston pumps use springs for their bias spring, control piston and pressure compensator. The bias spring attempts to keep the pump at full displacement and the control piston is pressurized by the compensator to reduce displacement until downstream pressure stabilizes.
Hydraulic cylinders employ the most powerful springs, sometimes to the tune of tons of compression force. Used to either extend or retract the cylinder in the absence of opposing pressure, they simplify the controls or perhaps provide an emergency actuator during a loss of power.
Hydraulics require precise springs with uniform load distribution, consistent surface contact and high stability. Open-ended springs tend to wobble or tilt when installed, so unless the valve is a low-performance version, they’re rarely used. The opposing ends of the spring are best when flat, providing a stable surface with even load distribution.
Depending on the material, wire diameter, height and number of coils, the strength of any given spring could vary widely. Some applications, such as a low-cracking pressure check valve, make do with a more delicate design, while high-pressure, direct-acting relief valves require a much stronger spring.
When selecting a spring, you must first know your load constant, which is often described as force per unit of distance, such as pounds per inch. This describes how many pounds of force are required to compress the spring one inch. For example, 10 lb/in. describes a spring that suits an application needing 20 lb of force at 2 in.
Other important factors are free length, wire diameter, OD and ID. Each of these plays a role in where any given spring can be installed and how much a spring can compress before reaching solid condition. If your application requires a particular minimum height, be sure to tell your spring supplier. In some cases, with limited height, you may need nested springs to provide extra force.
Any good spring supplier can also offer extended services such as performance curves and fatigue testing. A bench test to measure the load constant will ensure you get the spring you asked for, and any good manufacturer will be happy to test for fatigue life. Any spring can be good when new, but how it performs after thousands of cycles could be anyone’s guess.
Filed Under: Components Oil Coolers, Engineering Basics, Related Technologies, Technologies
