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What factors should be considered when selecting linear actuators for vertical operation?

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What factors should be considered when selecting linear actuators for vertical operation?

Here are five factors that designers and engineers should consider when choosing linear actuators that work vertically.


●Loading

Since the actuator will resist gravity during the upward stroke and will be assisted by gravity during the down stroke,the load in each case will be different,regardless of the process(e.g.,pressing,pulling or transporting).This affects not only the bearing life of the guide rails and drive mechanism, but also the required motor torque. In addition, the gravity on the load during start and stop affects the inertia of the system,which also plays a role in both actuator selection and motor sizing.


In vertical applications,it is often recommended to use two bearing blocks on each guide rail.This ensures that the assembly can support the pitch and yaw moment loads generated during acceleration and deceleration.Similarly,using an actuator with two parallel guides will allow the actuator to handle any rolling moments caused by uniformly undistributed loads or by external forces due to the process.If space permits,the best option is to choose an actuator that uses two rails with two bearing blocks each.


Vertical applications experience different load conditions in up and down strokes. 

Vertical applications experience different load conditions in up and down strokes.


●Buckling load

When using ball screws or lead screws in vertical applications,the full load manifests itself as an axial force,which can cause the screw to bend and eventually buckle under load.The buckling load is determined by the root diameter of the screw,the overhead length and the end bearing arrangement.The end bearing arrangement has a significant effect on the buckling load.The more firmly the screw assembly is secured,the higher the allowable flexural load.

Fc = maximum compressive load (N) 

Fc = maximum compressive load (N)

fb = end bearing factor

d1 = root diameter of screw (mm)

L = unsupported length (mm)


Placing the screw with the fixed bearing on top will put the screw under tension and avoid excessive compression on the screw.Most ball or lead screw actuators are constructed with a fixed bearing at the driven(motor)end,but it is important to check the bearing arrangement and make sure the actuator is mounted in such a way that the fixed bearing is at the top.


●Back driving

For safety reasons,screws are preferable to belts or linear motors in vertical applications,because they help prevent a load from catastrophically crashing if there’s a loss of power to the motor.However,they can and usually do reverse drive, depending on the friction,lead angle and efficiency of the screw.(In general,screws have a low tendency to back drive due to lower efficiency.By calculating the back drive torque and comparing it to the friction of the assembly(resistance of the nut,friction of the seal and friction of the end bearing),you can determine the likelihood of the screw assembly driving backwards.If the back drive torque is less than the friction of the assembly,it is unlikely that the load will cause backward drive.


Tb = back driving torque (Nm) 

Tb = back driving torque (Nm)

F = axial load (N)

P = screw lead (m)

η2 = reverse efficiency (0.8 to 0.9 for ball screws)*


*The efficiency of driving behind is usually lower than the efficiency of normal operation.Be sure to check the manufacturer's rear-drive efficiency specifications.


●Lubrication

Most bearings and actuators have lubrication ports on all four sides,primarily for ease of access in the event of an obstruction blocking one or more ports.For some designs,the orientation of the assembly also determines which lubrication ports should be used.Since gravity is an obstacle to lubricating the upper rail of a linear guide or the upper race of a screw,it is especially important to check the manufacturer's lubrication guidelines for vertical installations.In many cases,manufacturers recommend against the use of oil in vertical applications.For grease lubrication,assemblies typically have specially metered lubrication paths to ensure that the grease reaches all critical surfaces.


Using the correct ports is critical for ensuring that linear guides and drives are properly lubricated in vertical applications. 

Using the correct ports is critical for ensuring that linear guides and drives are properly lubricated in vertical applications.


●Contamination

The advantage of vertically oriented actuators is that liquid contaminants are usually drained,reducing the risk of corrosion.However,very fine,light particles,such as glass fibers and ceramic powders,are more likely to adhere to the bearing surface and be "pushed" by the end seals,causing contamination to build up and increasing the risk of entry into the bearing.To ensure best protection in vertical applications,use linear guides with front and side seals.In addition, choosing an actuator with its own sealing or covering mechanism(preferably a full contact seal,not just a cover)provides additional protection against air and liquid contamination.

 


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