FAQs - Electric Linear Actuators

Find an answer to some frequently asked questions about our range of products. From Technical specifications to operational issues to clarification of terms.

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  1. What is a Linear Actuator?
  2. A linear actuator is a device that moves between two points in a linear motion. Linear actuators are available based on a variety of technologies including: Mechanical, Electro-Mechanical, Direct Electric (Linear Motors), Hydraulic & Pneumatic.

    All of Power Jacks linear actuators are either mechanical or electro-mechanical. Electro-Mechanical Linear actuators are also known as Electric Linear Actuators due to the primary drive being an electric motor.

    Electric Linear Actuators (ELA) convert the rotary motion of the electric motor to linear motion by means of two main components a gearbox (rotary motion) and a lead screw (linear motion).

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  3. What Are The Main Features of a ELA?
  4. An Electric Linear Actuator can be split into its two main sections:

    1. Drive Unit
      a) Gearbox and motor.
    2. Actuation Unit
      a) Ram section of actuator.

    The drive unit typically comprises of a gearbox and motor. Although some actuators are direct motor driven with no gearbox. The gearbox is either a single or double stage gear set as standard (helical, bevel, spiroid, worm, or helical-worm type gearing). The actuation unit is based around a lead screw assembly, which converts the rotary motion into linear movement. As the lead screw rotates, the nut extends and retracts the ram, which is attached to the load. The lead screw assembly is packaged inside a ram arrangement with guides and seals to provide a sturdy enclosure.

    Three types of lead screws are used in electric linear actuators: Machine Screw, Ball Screw & Roller Screw.

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  5. How Does an Electric Linear Actuator Operate?
  6. The electric motors rotation drives the primary gear of the gearbox, which through a single or more gear stages turns the gearboxes output shaft (final gear). This rotates in unison with the lead screw, as the lead screw is fixed in place to the gear. When the lead screw rotates the lead nut that mates with the lead screw translates along the screw and so converts rotary motion to linear motion.

    The lead nut is fixed to the “ram” or “inner tube” so this item translates with the nut. Note that for linear motion to occur the ram (including lead nut) must be restrained from rotating with the lead screw. This is usually done by fixing the end of the ram via its end connection to the object that needs to be moved. If the object is in free space then the actuators ram needs an anti-rotation device fitted. The outer tube seals the lead screw into the ram which protects the screw from dirt, debris and damage and acts as a lubrication store. In addition the exposed portion of the ram can have a flexible covering attached to provide extra protection (e.g. bellows boot).

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  7. What is The Difference between Static Load and Dynamic Load?
  8. Dynamic, working, or lifting load is the force that will be applied to the linear actuator while it is in motion. Static load, also called the holding load, is the force that will be applied to the linear actuator when it is not in motion.

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  9. In What Direction can Loads be applied to Linear Actuators?
  10. Linear actuators can be used in tension, compression, or combination applications. Eccentric and side loading should be avoided. Please consult technical catalogues to ensure that all hardware used in conjunction with the linear actuator can withstand the maximum restraining torque.

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  11. Is Side Loading and Eccentric Loading Permissible on Linear Actuators?
  12. Side loading, or radial loading is a force applied perpendicular to the linear actuator centreline. Eccentric loading is any force whose centre of gravity does not act through the longitudinal axis of the actuator. Both side loading and eccentric loading should always be avoided as they can cause binding and shorten the life of the linear actuator.

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  13. Can Linear Actuators Have Limit Switches?
  14. Linear actuators can be supplied with limit switches. The type of limit switches available varies with each product range:

    • EMA Series linear actuators
      Have the standard option of electro-mechanical limit switches. Other limit switch types such as inductive proximity, magnetic proximity, etc.. can be supplied on request, for details consult Power jacks.

    • Rolaram linear actuators
      Have the standard option of inductive proximity limit switches. Other limit switch types such as electro-mechanical, magnetic proximity, rotary cam etc.. can be supplied on request, for details consult Power jacks.

    • Limit switches
      Are not pre-set on actuators. Limit switches allow you the flexibility to set the limits of travel on your actuator to fit your particular application. Easy to follow instructions are included in the installation manual, consult Power Jacks if further assistance is required. The customer is responsible for properly setting the limit switch in the unit. If the limit switches are not set, or are improperly set, the unit may be damaged during operation. In addition, limit switches may require resetting if the translating tube of your linear actuator is rotated manually, as this will change the limit switch setting.
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  15. What Type of Motors do the Linear Actuators Use?
  16. All of the linear actuators are available with AC or DC motor variants, however each range has preferred standard types.

    EMA actuators
    Are available with 220-240 VAC 1-phase motors, 220-240/380-415VAC 3-phase motors (50/60Hz) or 24VDC motors.

    Rolaram linear actuators
    Are available with 220-240/380-415VAC 3-phase motors (50/60Hz) as standard. Other motors such as 220-240 VAC 1-phase motors, or DC servo motors are available on request.

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  17. Are the Linear Linear Actuators Available in Different of Speeds?
  18. EMA-Series Linear Actuators
    are available in a variety of linear speeds and a standard list is detailed in the EMA Linear Actuators brochure (P11). Other speeds are possible, consult Power Jacks for advice. EMA units with variable speed rated motors are also available.

    Rolaram Linear Actuators
    are available in a wide variety of linear speeds and a standard list is detailed in the Rolaram Linear Actuators brochure. Other speeds are possible, consult Power Jacks for advice. Rolaram units with variable speed rated motors are also available.

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  19. What is the Duty Cycle Capability of a Linear Actuators?
  20. Duty cycle rating for a linear actuator is generally expressed as a percentage of “on time” (the ratio of on time to total time) or as distance travelled over a period of time. may also be expressed as a percentage, The duty cycle rating is expressed differently for different actuator types:

    EMA-Series linear actuators
    Type “I” is rated as intermittent to 10 hours running per day, with less than 10 starts per day. Type “C” is rated as continuous duty. Type “H” is rated as continuous duty, high speed.

    Rolaram linear actuators
    are rated up to continuous duty. Duty cycle and continuous duty ratings of the linear actuator should not be exceeded. To do so could cause damage to the unit thus voiding any warranty on the motor. See instruction and maintenance sheet for these ratings or consult Power Jacks Ltd. When considering the duty cycle of the unit the life of the linear actuator should also be considered.

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  21. What Type of Mounting Do the Linear Actuators Have?
  22. The linear actuators generally have a mounting option at the end of the ram and at the gearbox end of the actuator to allow a pivoting movement. EMA series linear actuators have double clevis options as standard, where as the Rolaram has a clevis to trunnion option as standard with double clevis available on request. However all have multiple mounting options including clevis, trunnion, fork, top plate, base plate, threaded end, etc.

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  23. Are Clutches Available on the Linear Actuators?
  24. Clutches/torque limiters are available on most of Power Jacks linear actuators whether as a standard option or a custom design.

    The EMA-Series linear actuator have a standard friction clutch option. The friction clutch in these actuators is set to slip when the rated load limit of the actuator is exceeded. This is to prevent damage to the actuator due to jamming or over-heating resulting from an excessive load. The load will be held securely should the clutch slip. The clutch also allows end of travel protection, but is not designed to be slipped repeatedly. Select an linear actuator with internal limit switches or install external limit switches. Severe over-loading will cause the actuator clutch to slip and allow the load to self-lower.

    Rolaram linear actuators can have a clutch or torque limiter fitted on request. These clutches can be specified to meet any requirement e.g. ratcheting auto re-engagement, overload manual re-engagement, integrated limit switch, etc.

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  25. What Type of Enclosures do the Linear Actuators have?
  26. EMA-Series linear actuators
    are IP54 (NEMA 3) as standard with options for IP55, IP56 and IP65 (NEMA 4) available.

    Rolaram Linear Actuators
    are IP54 (NEMA 3) as standard with options for IP55, IP56 and IP65 (NEMA 4) available.

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  27. Is Back-driving Possible in Electro-mechanical Linear Actuators?
  28. Unless otherwise stated back-driving is possible in all electric Linear Actuators. Actuators that use a ball screw or roller screw as the lead screw have an electrical brake (typically motor mounted) to prevent the load from back-driving the actuator. Note if a machine screw actuator is considered self-locking, it may still back-drive if significant vibration and cyclic temperature variations are present.

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  29. Can an Linear Actuators be run into a Dead Stop?
  30. Linear Actuators are not recommended for use in applications where it they are run into dead stops or can be jammed. Examples of jamming include over-travelling the limit switches and jamming the nut and screw internally at the extreme ends of the stroke, or driving the actuator against an immovable object and thus overloading the actuator severely. Therefore consult Power Jacks if jamming is expected. Power Jacks can provide solutions for actuators which are required to exert a force against an end stop, for example an actuator fitted with a spring damper device where spring travel is limit by a limit switch.

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  31. What are the Common Factors in the Failure of a Linear Actuator?
  32. Improper loading, failure to set limit switches excessive duty and extreme environments may contribute to premature actuator failure.

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  33. Can two or More Linear Actuators be Synchronised?
  34. If actuators are run using a “Direct On-Line” (DOL) starting method then they can only be roughly synchronised for speed and position. Small differences in motor speed and actuator loading may cause the units to get out of synchronisation. The units can therefore not be guaranteed to run in synchronisation. For exact synchronisation a closed loop control system is recommended. This can be achieved using AC motors with AC inverters or Servo motors with a matched drive as part of a control solution. The speed and position feedback is typically provided by an incremental or absolute encoder. For further advice consult Power Jacks.

    Alternatively EMA Linear Actuators and Rolaram Linear Actuators can be manufactured so that they can be mechanically connected with drive shafts with a single motor driving the connected unit.

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  35. Can the Linear Actuators be Supplied with Corrosion Resistant Properties?
  36. The EMA Linear Actuators and Rolaram Linear Actuators can be supplied with alternative materials and/or paint specifications for high corrosive areas. These options include stainless steel, chrome plating, Electro-nickel plating, epoxy paint, etc.

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  37. What Type of Lubricants do the Linear Actuators Use?
  38. The EMA series linear actuators are grease lubricated for the ram and gearbox assemblies. The Rolaram linear actuators typically have a grease lubricated ram and oil lubricated gearbox, however this is reviewed for high duty applications where a totally oil lubricated unit may be required. The EMA and Rolaram linear actuators can both be supplied with industry specific lubricants, such as food or nuclear grade grease.

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