# Off-centre actuation

I'm trying to lift a platform with several guides using a linear actuator, but off the centre as follows:

The load on the platform will be around 100kg, which is just within the linear actuator's 120kg load capacity. The platform will be around 50cm*50cm. My plan is to use 2-4 rods as guides with linear (or tapered?) bearings at each corner (or opposite sides if 2 rods) of the platform. Does anyone know if this would be a good idea? Would the linear actuator wear out more rapidly if the actuation was off-centre?

Any advice appreciated.

## 3 Answers

Does anyone know if this would be a good idea?

The proposed design has poor rigidity towards torsional loads. It doesn't matter whether the actuator is off-center or if the load is off-center: if load's center of gravity and actuator are not at the same position, the torsional load will likely damage the bearings.

In your example with two guide rails, torsional loads try to rotate the two linear bearings. The bearings are not usually designed to take much load in this direction, and some are in fact designed to be compliant of small rotations (alignment errors).

You can think of it like this: the actuator force is pulling on a long moment arm, while the bearing forces have a much shorter arm. Therefore the bearing has to handle many times larger force than is actually needed to keep the platform up. In your case of 100kg load, the bearing force would likely be hundreds of kilograms.

The example with four guide rails is slightly better in that it has half the load per bearing because there are four of them. Because they are all in the same plane, the lever advantage is equal as with two guide rails.

Would the linear actuator wear out more rapidly if the actuation was off-centre?

If the linear actuator is rigidly connected, it might take some of the torsional force on itself and wear out its internal sliding surfaces more rapidly. But the best practice is to connect linear actuators in a way that does not transmit torque. In fact, most linear actuators (random example) have only a single mounting hole at each end. Such a mounting prevents premature wear on the actuator - it would be the guide rail bearings that would break or wear out.

kamran's suggestion of a scissor lift mechanism is a good choice, especially considering the quite large 100 kg load. The diagonal arms of a scissor mechanism transfer the torsional loads to the support of the scissor arm ends.

There are however other alternatives also. For example, one can mount four bearings on two guide rails like in the picture below. Now the moment arm for bearing forces is approximately the same as the moment arm for load forces, and horizontal load on bearings is thus smaller. The actual location of the actuator is not very important, as it only requires correctly sizing the guide rails and bearings to handle the load.

• Thanks. Can I ask how the distances between the bearings should be properly determined? Are they done by hand calculations or CAD simulation? Sep 21 '20 at 5:31
• @JohnM. Hand calculations are sufficient to calculate the loads on the bearings. I would start the design by making vertical distances as large as the design can accommodate, and then calculate how strong bearings and guide rails are required.
– jpa
Sep 21 '20 at 5:56
• For a nice example of an actual implementation: hackaday.com/2020/12/28/build-your-own-custom-elevator
– jpa
Dec 30 '20 at 4:49

Your mechanism is going to lock up like the old car jacks immediately, due to differential raising of the platform. I am sorry but it looks like a textbook example of what not to do.

in most real-life cases where the lifting actuator has to be off-center, they use a linked pair of scissors mechanism, like what they do in trucks' gate lifter, or the hydraulic scissor lifts. If you search for Hydraulic scissors lift, you will find one already made.

IMHO (and I guess you already know that), from a structural point of view putting 100 kg at 1/4[m] will create a huge amount of bending moment on a actuator that it is rated for 120[kg]. An although you don't clarify whether the actuator electric/hydraulic, if bearings are involved you are bound to get failure sooner rather than later. Normally this type of mechanisms, have (or should have) diagrams with the maximum rated force vs the offset from the center of the actuator.

Additionally, irrespective of the type of bearings you use on the posts, you are going to be very prone to locking/blocking of movements, if you use the design you present in the figure.

Also, in believe that, 2 posts located at the middle of opposing sides (your first image) will probably be better (slightly) than 4 in terms of not abusing the motor.

In any case, if you insist on off center, one solution I can think (definitely not the optimal), is the following:

In this case you turn upside down the piston, and secure it on the support. That way all the bending moments get "absorbed" by the support, and you get only axial loads on the Actuator (which is the intended use). I haven't drawn the posts here, but you could envisage only one other post in this case to the right, or create two to the plane normal to the screen (like your first design).