FREQUENTLY ASKED QUESTIONS
Electric Linear Actuators
Click on a question from the list below for more details:
What is The Difference between Static Load and Dynamic Load ?
Dynamic, working, or lifting load is the force that will be applied to the actuator while it is in motion. Static load, also called the holding load, is the force that will be applied to the actuator when it is not in motion.
In What Direction can Loads be applied to Linear Actuators ?
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 actuator can withstand the maximum restraining torque.
Is Side Loading and Eccentric Loading Permissible ?
Side loading, or radial loading is a force applied perpendicular to the 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 actuator.
Can Actuators Have Limit Switches ?
Linear actuators can be supplied with limit switches. The type of limit switches available varies with each product range:
- EM-Series / TracMaster actuators have either electro-mechanical limit switches or magnetic proximity switches. There are no limit switch options for these units. The limit switch type for each model is defined in the catalogue.
- EMA Series 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 Screw Jack.
- Rolaram 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 Screw Jack.
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 Screw Jack 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 actuator is rotated manually, as this will change the limit switch setting.
What Type of Motors do the Actuators Use ?
All of the actuators are available with AC or DC motor variants, however each range has preferred standard types.
- EM-Series / TracMaster actuators are available with 12VDC, 24VDC or 220-240 VAC 1-phase motors.
- EMA actuators are available with 220-240 VAC 1-phase motors, 220-240/380-415VAC 3-phase motors (50/60Hz) or 24VDC motors.
- Rolaram 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.
Are the Linear Actuators Available in Different of Speeds ?
- The EM-Series / TracMaster actuators have one motor and gearbox combination. The linear speed is therefore set for these units. The speed for each model will vary under load, typical lifting speeds at various capacities within the operating range of each actuator are graphed in the catalogue.
- EMA linear actuators are available in a variety of linear speeds and a standard list is detailed in the Design Guide PJLMPTDG-01. Other speeds are possible, consult Screw Jack 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 Design Guide PJLMPTDG-01. Other speeds are possible, consult Screw Jack for advice. Rolaram 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 Design Guide PJLMPTDG-01. Other speeds are possible, consult Screw Jack for advice. Rolaram units with variable speed rated motors are also available.
What is the Duty Cycle Capability of a Linear Actuator ?
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:
- EM-Series / TracMaster actuators - The Duty cycle, measured in inches per hour, is the total distance an actuator may travel (extension and retraction) in an hour with equally spaced intervals between each cycle.
- EMA-Series 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 actuators - are rated up to continuous duty.
Duty cycle and continuous duty ratings of the 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 Screw Jack Ltd.
When considering the duty cycle of the unit the life of the actuator should also be considered.
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What Type of Mounting Do the Linear Actuators Have ?
The actuators generally have a mounting option at each end of the ram to allow a pivoting movement. EM and EMA series actuators both have double clevis options as standard, where as the Rolaram has a clevis to trunnion option. However all have multiple mounting options including clevis, trunnion, fork, top plate, base plate, threaded end, etc..
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Are Clutches Available on the Linear Actuators ?
Clutches / torque limiters are available on most of Screw Jack linear actuators whether as a standard option or a custom design.
The EM-Series & EMA-Series actuator both 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 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 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..
What Type of Enclosures do the Actuators have ?
- EM-Series TracMaster actuators are general purpose industrial units with an enclosure rating of IP44. These units can have an optional weather-resistant treatment for outdoor applications.
- EMA-Series actuators are IP54 (NEMA 3) as standard with options for IP55, IP56 and IP65 (NEMA 4) available.
- Rolaram actuators are IP54 (NEMA 3) as standard with options for IP55, IP56 and IP65 (NEMA 4) available.
Is Back-driving Possible in Electro-mechanical Actuators ?
Unless otherwise stated back-driving is possible in all electric linear actuators. Actuators that use a ball screw or roller screw as the lifting 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.
Can an Actuator be run into a Dead Stop?
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 Screw Jack if jamming is expected. Screw Jack 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 a limit switch.
What are the Common Factors in the Failure of a Linear Actuator ?
Improper loading, failure to set limit switches excessive duty and extreme environments may contribute to premature actuator failure.
Can two or More Linear Actuators be Synchronised ?
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 an incremental or absolute encoder. For further advice consult Screw Jack.
Alternatively EMA and Rolaram actuators can be manufactured so that they can be mechanically connected with drive shafts with a single motor driving the connected units.
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Can the Actuator be Supplied with Corrosion Resistant Properties ?
The EMA and Rolaram 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..
What Type of Lubricants do the Actuators Use ?
The EM (TracMaster) and EMA series actuators are grease lubricated for the ram and gearbox assemblies. The Rolaram 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 units can both be supplied with industry specific lubricants, such as food or nuclear grade grease.
