When designing a system incorporating actuation it is necessary to decide which type of actuation system best suits the technical and economic demands of the system and the cost of running and maintaining the system over its whole life cycle.
Mechanical screw jacks and linear actuators are increasingly becoming the preferred choice of actuation over the equivalent hydraulic or pneumatic systems with advantages including simpler and smaller installation, a cleaner, healthier, quieter environment, lower energy costs and less maintenance.
There are compromises and considerations to be made when choosing between the various systems for a particular application and each has its own benefits. For example, the cost of an individual hydraulic cylinder may be lower than that of a screw jack but the cost of the whole hydraulic system may be greater than the electromechanical equivalent. This is more likely to be the case where only a small number of actuators are required.
Here we look at the pros and cons of the various options.
Screw Jacks (and Linear Actuators):
A typical modular screw jack system with trapezoidal (acme) lead screw may consist of a number of screw jacks connected to gearboxes and driveshafts to provide very accurate, reliable lifting/positioning solutions. Due to the system architecture screw jacks are inherently clean in their operation with no contaminants to be released to their environment.
Screw jacks are suited to repetitive motion in a fixed configuration and as such, these systems are used throughout the world to lift, lower, push, pull, tension and pivot loads and are found in markets including steel works equipment, the food & beverage industry, water processing, Pharmaceutical, Medical and Laboratory Equipment, Packaging Equipment, Nuclear and Aerospace and General Mechanical Handling.
A Kelston 4x Screw Jack System – The lifting nuts translate and are fixed to a common load:
A screw jack is likely to be more expensive than a hydraulic cylinder but an electromechanical system is simpler to install than the equivalent hydraulic system and maintenance costs are low. Further cost benefits may come from the fact that power is only required when the system is in operation. This differs from hydraulic and pneumatic systems which require a pump being in constant use to maintain a force/pressure.
The contact between the screw threads leads to frictional losses during operation and therefore, from an energy usage point of view, screw jack systems may be suited more to installations that require non-continuous or intermittent use. The life of an trapezoidal screw jack system is dependent on the rotational speed of operation, load on the contacting gear faces and lubrication. They are not appropriate for continuous operation situations due to the friction in the system and would not be suitable, for example, in a pneumatic drill which requires a continuous, fast, back-forth motion.
This friction, however becomes an advantage if there were to be a power outage since these systems are inherently self-locking under static load and often also under dynamic load (depending on load, vibration and the screw thread angles). This differs to the alternatives which require an additional braking system to prevent uncontrolled motion that may cause damage and injury.
To showcase the fact that electromechanical can replace hydraulic systems, SKF built a prototype forklift truck replacing the traditionally used hydraulic systems that control the movement of the forks and the steering with electromechanical actuators. Potential benefits include reduced maintenance, safety and impact on the environment since potentially harmful high pressure hydraulic fluid is removed.
As opposed to a screw jack with an trapezoidal lead screw, they are also available with ball screws. Although more expensive, they have the advantage of being much more efficient due to very low friction and lend themselves well to continuous operation. They are not, however self-locking. Kelston designed and produces ball screw jack electromechanical actuators for AB Dynamics for driving their full vehicle Suspension Parameter Measurement Machine. This kind of test equipment is traditionally hydraulically driven and is an example of electromechanical actuators increasingly replacing hydraulic systems.