Heat Environment

A major problem for hydraulic systems is heat management. The inefficiencies of hydraulic systems cause overheating. Hydraulic fluid temperatures above 80°C damage most seal compounds and accelerate degradation of the oil. Circuit design can play a big part in generating heat. Hoses, fittings and other components can be a factor if they are inaccurately sized for the flow requirements. Sophisticated load-sensing systems can mitigate most heat issues, but can be costly. Heat exchangers are an economical way to manage this without significantly over-sizing your reservoir and ultimately your HPU footprint.

There are two ways to solve overheating problems in hydraulic systems: decrease heat load or increase heat dissipation. Hydraulic systems dissipate heat through the reservoir, by having the correct level of oil or use of a heat exchanger. Keeping a hydraulic system in proper form to manage heat will require constant attention and maintenance.

Electric actuators can overheat at times, but the cause is usually due to extreme changes in duty cycle or running the actuator harder than it was designed or sized for. Extreme changes in the ambient temperature not accounted for in the design may also cause overheating, but these conditions are exceptions and typically would not occur in most factory automation environments. Due to their higher efficiency and sizing accuracy, electric actuator systems can be selected to run at a desired temperature for the given amount of work required. Accurately predicting temperature allows the electric actuator system to perform consistently without affecting the life of the device.

Cold Environment

Cold temperatures present other problems for hydraulic systems. Cold oil causes sluggish and inconsistent operation until the oil is warm—which in turn causes large swings in force and speed. Hot to cold temperature swings can also affect the integrity of the rod seal—a critical component that helps to prevent leaks and contamination. Oil tank heaters can manage start-up temperatures, but they are an extra component and expense.

Electric actuators, on the other hand, can be deployed with extreme temperature grease which allows for quick, effective starts in cold temperatures. There is usually a small performance difference in force repeatability from a cold to a warm temperature. This difference is acceptable in most applications, often making it preferable to choose electric over hydraulic actuators.

Service Life and Maintenance

Hydraulics are rugged, widely used devices that can have a long service life. The trade-off is that they require frequent maintenance and attention to achieve the desired performance over the life of the system. Maintaining the integrity of the rod and piston seals is key to repeatability because these are the main elements that contain the pressure required for motion and force. If these components wear or are damaged, they must be replaced. Otherwise, force will be reduced at initial pressure or “blow by” will affect speed. Changing oil filters and oil on a periodic basis are additional maintenance tasks. Contaminated or degraded oil can also severely affect the operation of the system. Neglecting maintenance items will lead to leaks or contamination and premature failure of components.

On the other hand, electric actuators sized for the life of an application require no maintenance.  The main power elements of an electric actuator are the power screw and other thrust-bearing elements.  These elements have a Dynamic Load Rating (DLR) specification that can be utilized to estimate the amount of work (force over distance) an electric actuator can achieve.  Utilizing an industry standard L10 life estimation, electric actuators can be estimated with 90% reliability to last in an application.

These power elements are typically greased for life, but easy, in-field greasing methods can be applied if necessary. The other wear element on an electric rod-style actuator is the rod seal, designed to hold out water, dust and other environment contamination from the actuator’s internal components. Unlike hydraulic cylinder seals, there is no pressure to hold in the actuator for proper operation. Even if the seal fails, the actuator will continue to operate. Rod seals on most electric actuators are easily and inexpensively replaced if damage occurs. Misuse is the primary reason that electric actuators fail. Exceeding the actuator’s performance specifications for extended periods of time or gross neglect are the most common misuse factors.