Knowledge Base

Electromechanical vs. Hydraulics

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 with 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:

Screw jacks, screw jack, screwjacks, Kelston, Bristol

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.

AB Dynamics full vehicle Suspension Parameter Measurement Machine utilising Kelston Ball Screw Jack Actuators:

Kelston Linear Actuator

Hydraulic and Pneumatic Systems:

In contrast to the fixed configuration of screw jack systems, hydraulic and pneumatic systems can accommodate flexible hoses between a remote power source and the required work tool. This becomes useful in situations where weight is of an issue such as a manual pneumatic drill or in mining/drilling operations where the power and flexibility of a hydraulic system are of importance.

Hydraulic systems can achieve the greatest forces of all such systems with force capabilities ~25 times that of pneumatics. Include with this the flexibility of the pressure hoses which allow for articulation of the machinery, these systems lend themselves very well to heavy duty industry such as drilling, mining digging etc.

As well as pneumatic drills, pneumatic systems are generally the ‘go to’ for applications that require moving relatively light load between two positions. They offer a combination of relatively low initial cost and high speed making them a good choice for simple motion applications.

As can be seen in the following images, hydraulic and pneumatic systems offer flexibility and articulation which is important in many situations.

Manually Operated Pneumatic Drill:

Flexible Hydraulic Piping

There are a considerable number of separate components and assemblies required for hydraulic and pneumatic systems:

Hydraulic systems include:

  • Electric motor/pump
  • Hydraulic pump
  • Hydraulic reservoir
  • Valves
  • Operator controls
  • Relays
  • Safety braking system
  • Hydraulic cylinder

Pneumatic systems include:

  • Compressor
  • Air/water separator
  • Filters
  • Dehumidifier
  • Pressure regulator
  • Lubricator
  • Control valves
  • Pneumatic cylinder

The ancillary equipment of these systems adds considerably to the complexity. With hydraulic systems, the fluid itself is also an important consideration. It may not be appropriate to use a conventional hydraulic fluid but one which is biodegradable or, due to fire or environmental considerations, a water-based fluid. There is a requirement for this fluid to be checked regularly for its lubricating properties, particle and water content. The viscosity of the fluid can alter considerably with ambient and working temperature fluctuations thus resulting in non-linear performance.

With respect to energy cost, electromechanical systems have a further advantage. The conversion of electrical energy to hydraulic or pneumatic pressure comes with energy losses. Additionally, for much equipment including plant, a high pressure must be kept in reserve for the requirements of peak operating load which may be intermittent.

Whereas hydraulic systems can hold a constant pressure without exerting additional energy, pneumatics, due to the compressible nature of gas, additional energy needs to be supplied to maintain the load position. This implies that, efficiency-wise, these systems lend themselves to constant operation requirements.

The high internal pressures within a hydraulic system and the multiple joints and fixtures imply that fluid leaks are a maintenance issue and a common problem. Small leaks form an environmental issue. Large, sudden leaks can be a major safety concern with the escape of high pressure fluid. Additionally, a sudden loss in pressure, if there is no safety braking system in place, would result in uncontrolled movement of the end tool. This same negative safety characteristic can be applied to pneumatics also.

The tendency of hydraulic systems to leak impacts on the industry sectors that may employ hydraulic systems. For example, since the fluid may impact on the final product, the food and beverage industry are increasingly looking towards electromechanical systems. For this reason, Kelston cubic screw jacks are utilised on machinery used in the bread, biscuit and confectionary sectors.

Construction machinery almost entirely utilises hydraulic systems. One of the reasons for this and an inherent advantage of hydraulic (and pneumatic) systems is the resilience of the system to sudden shock loads for example when an excavator bucket hits a rock. There is some amount of compressibility of the hydraulic fluid that helps protect the excavators moving parts.

Summary:

A lot of negative aspects of hydraulic and pneumatic systems have been highlighted here but this is not meant to imply that electromechanical systems are more appropriate for all applications. Electromechanical, Hydraulic and pneumatic systems each have their own benefits.

The following table highlights the pros and cons of electromechanical v’s hydraulic and pneumatic systems:

System Type
Screw Jack or Linear Actuator Hydraulic Pneumatic
Costs Moderate initial unit costs. Low operating cost since power is only required during operation. Low maintenance costs. Low initial outlay if ignoring hydraulic power unit. High installation and maintenance costs. High energy usage due to requirement of pump being in constant use. Low initial outlay if ignoring pneumatic power unit. High installation and maintenance costs. High energy usage due to requirement of pump being in constant use.
Safety In the event of power loss, screw jacks and linear actuators can be self-locking. There is no chance of a high pressure oil leak. In the event of power loss or hydraulic leak, without an additional breaking system, the actuator motion is not controlled. In the event of power loss or pneumatic leak, without an additional breaking system, the actuator motion is not controlled.
Capacity Up to 1000kN. Extremely high. The most powerful option. Up to 25kN but usually considerably less.
Control Easily compatible with standard CNC. This allows for automatic operation of complex motion sequences. Compromised through requirement of electronic/fluid interface and valving. Control is complicated by hysteresis, supply pressure and temperature. Process is inherently non-linear. The compressible power source complicates servo control.
Environment Clean with no hazardous hydraulic fluid. Energy efficient. Temperature extremes can be a problem as seals are prone to leak. Required disposal of hazardous hydraulic fluid. Temperature extremes can be a problem as seals are prone to leak.
Configuration Non flexible - Fixed configuration. Flexible hoses/lines with remote power source allows for versatility. Flexible hoses/lines with remote power source allows for versatility.
Installation Simple electrical wiring. Plumbing, filtering and pumps required. Plumbing, filtering and pumps required.
Accuracy Very accurate and repeatable. Very accurate although position sensing and electrohydraulic valving is required. Has tendency to creep due to strains within system. Difficult to achieve since gas is compressible. Position sensing and electro-pneumatic valving is required. Has tendency to creep due to strains within system.
Life Millions of cycles are achievable at rated load which is easily calculated. Requires lubrication maintenance. Usually good. Self-lubricating. Dependant on design, seal wear and maintenance. Usually good. Requires aerosolised oil for lubrication. Dependant on design, seal wear and maintenance.

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