Whether you’re a mining operation, a construction company, a manufacturer or a farm, your hydraulic cylinders are the hardworking muscles of your operation. These faithful servants are employed to do everything from lift, pull, open, close and even crush depending on the application. As linear actuators, they have a simple role to apply a pushing or a pulling force in a straight line. However, there are many factors to consider when selecting the cylinder that is right for the job. These go beyond the push pull force required to perform the task to include cylinder mounting style, stroke length, speed, operating pressure, direction of force and other factors.
Selecting the wrong cylinder could lead to inefficiencies, premature wear or failure of the cylinder and or other parts of the system. Getting the best performance and durability out of your cylinder starts with asking the right questions. So, here are five of them to help you make a good call when it comes to hydraulic cylinder selection.
1. What do you need it to do? (Application)
The first step in determining the correct hydraulic cylinder is to assess the application. What task is the cylinder required to perform? Do you need the cylinder to push or pull or both? Will the cylinder deal mainly with compressive force? Will the cylinder need to hold a load in place? The job at hand dictates cylinder specifications, from bore size to stroke length to surface coating, all of which we will discuss in the questions that follow.
2. How much mass do you need to move? (Force)
Newton's first law states that every object will remain at rest unless compelled to change its state by the action of an external force. So, in order for any object to move, the first thing we need to know the amount of mass we need to overcome. But that's not the full story, because the way in which the mass is to be moved will have an effect on how much force is required to move it.
For example, lifting a load vertically requires more force than moving a mass pushed horizontally along the ground. Whereas crane cylinders, which push on the boom very far from the load require more force to overcome the lever effect. Conversely, if the load is supported by rollers or hinges, less force is required.
Thus, physics and geometry of the application have a great effect on the power required from the cylinder. This needs to be factored into the equation when determining the force required, which in turn, is used to calculate the bore size of the cylinder. As a general rule you should opt for a tonnage rating 20% above the force required for the application. That's because peak loads can be increased by factors such as friction on cylinder seals and bearings, as well as efficiency losses in the hydraulics.
3. How far do you need to move it? (Stroke Length)
Stroke length is one of the most important things to take into consideration when choosing the right hydraulic cylinder. The distance the cylinder rod must travel has an effect on the overall mechanics of the cylinder. As the cylinder extends, bending and buckling forces become magnified. It is therefore extremely important to size the rod diameter properly. Under-specifying the piston rod makes a cylinder more prone to stress, wear and failure. A fully extended rod can bend under its own weight and excessive bending leads to wear and damage to seals and bearings. In extreme cases, the piston could even become cocked within the bore leading to scoring and damaging of the the inner surface of the cylinder. The cylinder choice must accommodate for these things, including rod diameter, choice of materials, metal coatings, seals and bushes must be taken into consideration.
4. How does the cylinder need to be mounted?
There are many different mounting options and the correct choice is important for cylinder performance. This again depends on the application, and the mounting method you choose will first depend on whether the cylinder is stationary or will need to pivot.
Flange mounts are preferred for stationary cylinders, since these allow strong, rigid linear force transfer with minimal wear. However, they have little tolerance for misalignment.
If the application requires the cylinder to pivot - clevis, trunnion and spherical-bearing mounts allow the cylinder to change alignment as it extends. Clevis mounts are most often used for short stroke and small to medium bore cylinders. Clevis mounts with spherical bearings allow for a degree of misalignment, which helps improve cylinder longevity. For certain applications, a Trunnion mount may be suitable, which enables the cylinder to be mounted at different points length of the cylinder from head, mid and rear-mount versions as the application demands.
5. What environmental factors will the cylinder be facing?
Lastly, consider what conditions the cylinder is likely to encounter in its day-to-day life. Will it live in a clean environment or will it be exposed to abrasive dust or corrosive elements. Chrome plating is sufficient for most applications, but to handle working in extreme conditions, the piston rod must resist the corrosive effect of water, salt air and other harmful substances. In that case, stainless steel or advanced protective surfaces such as thermal-spray coating or plasma-welded overlay for corrosion protection and high wear resistance may be required. For some applications, cylinders must be able to withstand extreme high or low temperatures. Seals are probably the most vulnerable aspect of a hydraulic system and account for 90% of cylinder failures. Buna-N nitrile rubber seals can’t tolerate high operating temperatures. Viton synthetic rubber seals and fluorocarbon seals should be used for high temp applications.
Making the right call
As the hardest workers in any operation, correct cylinder selection is critical to keeping you performing flawlessly. The consequences for making the wrong choice range from premature wear in the best case, to complete cylinder failure. So, making the right call is all-important. Start by asking the right questions. The answers will guide you to choose a hydraulic cylinder with the right specifications - from the type of application to the size of the bore, length of stroke to selecting the correct seals and the type of surface coatings to stand up to the environment.