After reading this chapter, the student will be able to:
• Explain the construction and design features of hydraulic cylinders
• Describe in detail the operating principles of cylinders
• Explain the construction of various types of cylinders used in hydraulic systems
• Calculate the various cylinder performance parameters such as load-carrying capacity, speed and power
• Select and size cylinders for hydraulic applications
• Troubleshoot common cylinder problems.
A hydraulic system is generally concerned with activities related to moving, gripping or applying force to an object. Devices, which achieve these objectives, are referred to as actuators. Actuators are interface components that convert hydraulic power back to mechanical power. Based on whether an actuator gives rotational motion or linear motion, actuators are basically categorized as:
• Rotary actuators and
• Linear actuators.
Rotary actuators are nothing but motors that we have examined in the previous chapter. Linear actuators, as the name implies, are used to move objects or apply a force in a straight line. These are otherwise known as hydraulic cylinders.
Cylinders are linear actuators whose output force or motion is in a straight line. Their function is to convert hydraulic power into linear mechanical power. Hydraulic cylinders extend and retract to perform a complete cycle of operation. Their work applications as earlier discussed may include pulling, pushing, tilting and pressing. The type of cylinder to be used along with its design is based on a specific application. The simplest of linear actuators is a ram which is shown in Figure 5.1. It has only one fluid chamber and exerts force in one direction only. Rams are widely used in applications where stability is needed on heavy loads. Ram-type cylinders are practical for long strokes and are used on jacks, elevators and automobile hoists.
Hydraulic cylinders are further classified as:
• Single acting cylinders and
• Double acting cylinders.
Let us discuss these two cylinder types in detail from the design, construction and application point of view.