Actuators: Linear, Rotary

Consider a wheel loader or an excavator. Pascal's Law, which declares that pressure exerted on an enclosed liquid is transmitted undiminished in all directions at once, has been put to the test performing different functions throughout those machines.

Regardless of whether hydraulic fluid is being used to operate a cooling fan, articulated steering, or to propel the machine around the jobsite, the equipment doesn't start earning its keep until the bucket takes a bite out of a rock pile or a construction site. When that happens, an actuator converts the hydraulic pressure into mechanical force and motion.

It's with the actuator that the design of a circuit typically begins. With a good idea of what kind of force will be required to lift a work platform or a loaded scoop, designers specify the appropriate size cylinder and work back through the system.

There are many types of linear actuators to choose from, but the double-acting cylinder is the most commonly used in mobile equipment applications. In it, force is developed in either direction of the cylinder for extending and retracting the work.

Important as they are in the circuit, cylinders have traditionally been among the simplest component in the system. Lately, however, cylinders have started to increase in sophistication.

In the ongoing quest for longer-lived and more environmentally conscious equipment, hydraulic cylinders have been getting some special attention. New materials are being used in cylinder construction, and sensors have been installed to help the operator, as well as the machine, make appropriate decisions.

Steel tubing for the outer wall is popular, but composites that can withstand the high pressures of mobile equipment have shown promise for saving weight and fighting corrosion. New rod coatings provide wear and corrosion resistance in severe environments. Traditional chrome rod plating is one option, but many advanced coatings are also available.

Plus, as machine systems become more sophisticated, hydraulic actuators must be able to communicate with the machine.

A closer look

One of the largest waste hauling companies in the world is implementing two major programs. First is the company's goal to reduce its fleet's total cost of ownership (TCO) and customer service interruptions (CSI). Second, in October the company announced a major environmental initiative that includes spending up to $500 million over the next 10 years to increase fuel efficiency.

A significant component of the TCO program is to reduce maintenance costs on its fleet of garbage trucks. One of its largest maintenance expenses is the repair and/or replacement of hydraulic cylinders. The company's goal for new vehicle equipment is a life of at least 10 years without unscheduled hydraulic failures. Presently, after five years, garbage trucks are pulled from service for a complete rebuild, including replacement of hydraulic cylinders. The current goal is to achieve a five-year cylinder life without failure.

The company has found that once a truck body comes out of warranty and a cylinder needs to be repaired or replaced, these cylinders often failed within three months. It also found that there were no standards for the repairs being done, or for the replacement cylinders which were being purchased.

Parker-Hannifin's Mobile Cylinder Div. has been working with the company to help it meet its goals. With improvements in seals and cylinder design, Parker has been able to improve the life of replacement cylinders, which has significantly reduced equipment life cycle costs. Parker's involvement with the company's environmental goals includes helping it reduce vehicle component weight by the use of composites or structural polymers in cylinder manufacture, which can lead to vehicle weight reductions of more than 1,000 lbs. Reducing vehicle weight can help improve a vehicle's fuel economy.

While suppliers work with large fleets to help the companies meet their fuel efficiency and environmental goals, manufacturers are also depending more on the component supplier.