By Josh Cosford, Contributing Editor
The symbols for hydraulic motors, especially in their simpler forms, are very similar to those of hydraulic pumps. If you haven’t already had the chance, see my article on pumps symbols here first. The directional arrow that points inward to accept fluid power energy is the primary difference between pump and motor symbols, and in Figure 1 you can see the simple fixed displacement, unidirectional hydraulic motor.
The shaft symbol may or not be present in a schematic, as with the rotation direction arrow. The shaft symbol may even include a symbolic depiction of the device being rotated, such as a wheel or a drum. Always remember although there are ANSI and ISO standards for drawing schematics, the engineer or designer may draw a circuit as they wish, so you may come encounter modified or unknown symbols.
The second symbol stands for the variable displacement bi-rotational motor. The dark triangle depicting the direction of hydraulic energy is now diametrically opposed to indicate the motor takes fluid from both ports. If it’s not plain enough already, the shaft has bidirectional rotation arrows bent around the shaft as well. You will also notice both ports are now open to flow rather than one that terminates at the tank as the first symbol. Finally, the tell-tale variable arrow dissects the circle, showing us the motor has a variable displacement, although telling us nothing of how that might occur.
The final symbol of Figure 1 is just like the last, save for two slight differences. The dark flow triangles are stacked atop each other and in opposing directions. This configuration represents a unit capable of both pumping and absorbing hydraulic energy, or more succinctly, the variable displacement, bi-rotational pump-motor. Used in few locations other than a drive application, such as the clever hydraulic hybrid applications for dump trucks or loaders, where stopping energy can be fed back into the system and stored in an accumulator.
Motor controls, aside from hydrostatic drives, are not usually overly complicated. The variable displacement, bi-rotational hydraulic motor shown on the left in Figure 2 has everything the earlier one did save the case drain line. Being a simplified symbol, as most fluid power symbols are, it gives you basic details on what it does but doesn’t provide the scope of performance, the method of construction or dimensional envelope. The apparent mess of lines and shapes to the right does, in fact, break down the method of operation, at the very least.
By this point, the motor symbol needs no explanation, so we’ll skip that part. Denotations (a) and (b) are the work ports, which are the common characters used to denote work ports, even on the directional valves as well. Each work port terminates not only at the motor ports but also at the (c) component, which is called a shuttle valve. The shuttle valve is a 3-port check valve that always provides a flow path for the higher of the two work ports. In this case, work pressure is coming from port (a), so the check valve shuts off port (b) due to pressure differential.
The part looking like a spring-retracted cylinder can be considered as such, and this is the object primarily responsible for controlling motor displacement, which in turn will change torque and speed. The spring keeps the bias piston retracted, providing the motor with full displacement until told otherwise. How it gets told otherwise is through the pilot valve operated 3/2 valve shown at (g). In its neutral state, it provides a flow path to tank for the bias piston so that motor’s tendency is full flow.
When a pilot valve somewhere upstream of (x) is activated, fluid enters the pilot chamber of the 3/2 valve (g), where it shifts to provide pilot energy sourced from the shuttle valve (c) to the bias piston. The bias piston now shifts fully, reducing the swashplate angle to reduce flow. Just how low the displacement goes is dictated by the tiny stroke limiter (e), which is just an adjustment screw that prevents the swashplate from reducing its angle further. The orifice at (f) is used to dampen the actions of the pilot energy working to move the bias piston. Without this orifice, the pump may shift too quickly or be susceptible to work pressure fluctuations coming from ports (a) or (b).
A motor such as this might be used as a two-speed transmission. The full flow, large angle of the swashplate provides higher torque yet slower speed while shifting the pilot valve energizes the bias piston, reducing swashplate angle to reduce displacement, therefore increasing speed while lowering torque. The shuttle valve ensures pilot energy is available regardless of motor rotation direction, however, it should be noted that the pilot valve could instead be a mechanical lever or some sort of torque limiting valve.
The standard hydraulic cylinder depicted way back in Hydraulic Symbology 102 is as clean and pure as one can ask, but you’d be surprised at the number of ways a cylinder can be drawn (and therefore constructed). Where possible in these examples, I’ve made a wide piston rather than the single line in previous examples, and also shown the rod as a long rectangle as well. For some of these examples, it is required to make sense of the symbol, so I used it across the board for the purpose of consistency.
The Double Rod Cylinder is quite easy to understand. Instead of a single head, cap and rod, this component now has two rods joined by a common piston and is then guided through two heads. This symbol looks very much like its construction, at least from the point of view of the piston rod assembly.
The Spring Retract Cylinder is another that takes little to decode. It starts by looking much like a regular cylinder symbol, but now with a giant spring placed in the annular (rod side) area of the cylinder. Imagine now that pressure is applied to the cap side and the piston starts to extend. As it does, the spring compresses in an attempt to once again retract the cylinder without the help of rod side hydraulic energy. When pressure is relieved from the cap side, the cylinder retracts using the compressed energy stored within the spring.
Although more common with pneumatic applications, the double rod inside the Guided Cylinder still exits through a single head, which is machined for two parallel rods. The Guided Cylinder is used where rotation of the rod cannot occur, and is also called a Non-Rotating Cylinder.
A ram is a hydraulic cylinder with one fluid port on a tube that is stuffed with a rod. The rod is typically a large diameter relative to the body because the base of the rod also performs as the piston. So if your ram has a 4-in. rod, then your ram has a 4-in. piston. The port can be drawn nearly anywhere on a ram because fluid coming in the side will still extend the cylinder, as fluid pressurizing the rod radially has no effect on it.
Looking complex at first, the Single Acting Telescopic cylinder uses multiple stages of nested barrels acing as individual cylinders. Often installed rod side down (where the fluid power resides), these cylinders will extend with the largest bore area first (the big end), where it likely lifts the front end of a dump bed. With no other ports or springs to help with retraction, this cylinder counts only on the mass of the empty bed and gravity to retract.
When a load of a dump truck or other piece of machinery employing a telescopic cylinder goes over center (meaning past its pivot point), the load then pulls the rod out further. Gravity is no longer able to retract the cylinder and lower the dump body, so a Double Acting Telescopic cylinder is required. It’s very much like the single-acting example, but now with actual pistons and a complicated porting network. The symbol shows a piston rod within a piston rod within a piston rod, which fairly well represents how it is constructed.
Motors and actuators in real applications rarely employ some of the options shown, but they’re important to know so you not only understand schematics that may come across your desk. As well, know what components are available by way of their symbol allows for creativity and diversity in your schematic creations.