Frequently Asked Questions - Actuators for linear motion
1. Which model should I choose?
Servomech and Linear-Mech actuators are designed for industrial applications.
A lot of factors come into play when choosing a linear actuator. To summarise a few simple rules:
- For simpler applications, light to medium duties and lower costs, choose the LMR or small ATL types. The LMR has integrated limit switches. For heavier loads with integrated limit switches select the CLA.
- for higher forces and medium duties choose Acme thread ATL.
- for high duty cycles choose ballscrew actuators
- for high speed operation choose type UBA
2. What type of motor is best?
Lowest cost and suitable for the majority of applications are the standard IEC frames asynchronous motors. Three phase is more economic and gives better performance than one phase. DC powered actuators are ideal where there is no mains supply. As performance requirements rise, brake motors may be needed, particularly with type UBA which is reversible. Ultimately servo motors can be used for high dynamics and precision.
3. Acme thread or ballscrew designs? What duty cycles are allowed?
The choice depends primarily on the duty cycle you require. This is calculated as the running time over 10 minute period and acme thread actuators should not exceed 30% duty. However where the force required is less than the rated force of the actuator, higher duty cycles can be permitted. See page 22 of our Servomech catalogue download.
Ballscrew actuators are capable of 100% duty cycles.
4. Is backdriving possible? Are actuators self locking?
ATL type actuators use worm gearbox technology and this is a notoriously difficult area to be precise. Generally we define self-locking when an actuator will not move under a push or pull load applied statically. Also dynamic self locking is when an actuator stops on turning off the motor even under the effects of a load Full data on self-locking is given on page 25 of our servomech catalogue download.
In summary, most ATL and CLA actuators are statically self locking but may not be dynamically self-locking depending on the load. Most BSA actuators are not self locking and a brake motor may be needed. All UBA actuators are not self locking.
5. What speed accuracy do linear actuators achieve?
This depends on the type of motor. DC motors vary speed according to load. Depending on model, the speed may drop 20-40% compared to the rated speed as the load is increased. Graphs are available. AC asynchronous motors will show only a small speed variation with load. Typically 2-3%. Servo motors can hold speed very accurately depending on their control system
6. Does the stroke of the linear actuators need to be limited?
Yes - these industrial type actuators are not designed to be run into their internal limits and damage may occur. The use of limit switches that turn off the motor is recommended there are several alternatives.
- fit limit switches to an external mechanism
- small actuators type LMR & CLA have integral limit switches
- fit magnetic read switches to the linear actuator. These fit around the outer tube and sense position from a magnet that travels with the push rod. They are easy to adjust
- use an external limit switch device build onto the actuator. This is a standard option for ATL actuators and is described on page 94 of our Servomech catalogue download.
- with larger actuators use inductive proximity switches. These can be more economic for large actuators but are not adjustable.
High speed actuators may overrun limit switch position and an encoder position feedback should be considered
If there is a probability of external loads causing the actuator to jam mid-stroke, the use of a safety clutch should be considered.
7. What are the options for end fixings?
Hardened steel rod ends or hollow bore threaded end are the most economic solutions. Optional rear brackets are available to give pivoting connection. Care should be taken to ensure the end fixings do not apply any twisting loads to the actuator during the stroke.
Alternative front end fixings such as, ball joint and flange are available where they can assist with connection to the machine.
8. Why is anti-rotation required?
Linear actuators have a tendency for the push-rod to rotate rather than move linearly. This is not a problem with most applications where the load is not free to rotate. However if the load has no restraints for example raising a sign on a single actuator, you should specify the anti-rotation option.
9. Are there limits for side loads and buckling?
Generally side loads should be avoided. Buckling of the push rod is potentially a problem on longer strokes. On smaller actuators no problems are likely for strokes up to 350mm. Larger actuators can stroke without any possibility of buckling up to 800mm or more. Detail information is given in our Servomech catalogue download page 18. Note that pull loads are immune to buckling problems.
10. What lifetime can I expect?
Most applications with ATL/CLA actuators are low duty and consideration of lifetime is not relevant. Some ballscrew cases should be checked. Data is given on page 23 of our Servomech catalogue download and lifetime depends on the model, the force and the speed.
11. What are critical speeds?
For actuators with higher speeds and longer strokes, critical speeds for bending and whipping should be considered. Typically these come into effect beyond strokes of 500 mm and for linear speeds above 30mm/s. There are no critical speed problems with pull loads.
12. Can I position with linear actuators?
There are a number of options depending on the dynamic performance and accuracy required. A simple positioning system can be achieved by adding an inverter such as the Lenze 8400 Highline to an actuator with an ac asynchronous motor. This will ramp up and down and stop after a preset distance of movement. Higher accuracy can be achieved by adding encoder feedback. The encoder can be mounted to the actuator or the motor. Feedback to an 8400 Highline inverter will allow positioning with medium dynamics and accuracy depending on the application. Highest positioning accuracy can be achieved with servo motors and drives. Very high dynamics and accuracy are possible with ballscrew actuators. Technical advice should be sought on dimensioning the system.
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