# Electric motor power needed to move an scissor jack?

I'm trying to build a device that, on receiving a signal, lifts the mattress from one side (in a "slowly kick you out of bed" manner). Since linear actuators are generally expensive and i'm on a tight budget, I though of using a cheap scissor jack (like this one ), and move it with a simple electric motor, or most likely a gear reducer motor, but i'm no engineer (well technically a software engineer but that doesn't help here) so i don't know where to even begin to do the calculus. ¿Could anyone give me some pointers? I can do the math but i'm lost on the physics part.

PD: This is also my first time building any device, so if i'm missing some painfully obvious solution to this problem please tell.

• I'll take a liberty to propose a contrarian approach. Instead of gently lifting on side of the bed, gently lower one [the opposite] side of the bed. No motor required. – Nick Alexeev Mar 26 '17 at 18:49

This sort of thing comes up a lot.

For a low speed high load application like this the specification you are most interested in in torque. Essentially this is the sum of two components 1) overcoming the friction forces in the system 2) accelerating the mass you want to move.

In practice it's not that easy to calculate this sort of thing from first principals and it is often best to start with a ballpark estimate based on something similar which already exists.

With a scissor jack the fact that it is, by default manually operated helps a bit as you can easily set up a practical test eg set up an equivalent load situation and use a spring balance etc to find the minimum force which needs to be applied to the crank (and thus torque) required to get it moving. Anything more than that will give you extra margin as I don't imagine you require particularly snappy acceleration for this application.

I would also suggest that you might want to consider over-specifying the motor a bit and using speed control as it is going to be tricky to hit the exact speed you want just from calculations.

As an initial estimate it is possible to jack up a car with a screw jack and a cordless drill so you are looking at something in the 500W sort of range.