Newbie- Why Belt-Driven Lead Screw?

[Stuey]

I've seen a few designs where stepper motors are indirectly coupled to acme or ball screws with a belt. Why is this done?

A half hour of searching didn't turn up anything helpful.

One rotation of a directly coupled stepper results in one rotation of the dreive screw. With a belt, one stepper rotation results in say, 1/4" rotation of the drive screw. So is the belt intended to increase the precision of the machine? Or is it done for greater simplicity and efficiency?

[CarveOne]

Originally Posted by Stuey I've seen a few designs where stepper motors are indirectly coupled to acme or ball screws with a belt. Why is this done? A half hour of searching didn't turn up anything helpful. One rotation of a directly coupled stepper results in one rotation of the dreive screw. With a belt, one stepper rotation results in say, 1/4" rotation of the drive screw. So is the belt intended to increase the precision of the machine? Or is it done for greater simplicity and efficiency?

The timing belt drive system allows use of different size timing belt pulleys to give more speed, or more power. If the same pulley size is used on the motor and lead screw it is a 1:1 ratio just like directly driving the motor to the lead screw. As with with any other drive system there are advantages and disadvantages to deal with when using belts.

CarveOne

[Stuey]

I suppose that makes sense. I mostly see these on smaller machines.

[eSilviu]

Originally Posted by Stuey I suppose that makes sense. I mostly see these on smaller machines.

it may be also a cheaper way to connect motor to the acme thread (flexible couplings are avoided)

[CarveOne]

Originally Posted by Stuey I suppose that makes sense. I mostly see these on smaller machines.

That's because the longer the belt is, the more sag it has unless you add idler pulleys to help support the belt without having to tighten the belt really tight make the belt support itself. Then the belt can stretch over time and it will sag again. It also puts a heavy load on the bearings. When the belt has sag, it has backlash because it has to take up the slack in each direction it runs. Some types of belt construction has less sag per unit of distance. Those tend to be more expensive. Sagging belts also tend to jump up and down in mid span as the direction is reversed.

Belt drives can work very well when the right type and width of belt are used.

CarveOne

[Andre' B]

Power is constant, limited to what the motor can put out (RPM * torque) but for what is lost to efficiency.
What a belt allows is to trade RPM for more torque or torque for more RPM. It can also be used to trade speed for steps per inch but the designer needs to remember it also effects the torque available which the screw converts to linear force.

The fancy word is impedance matching, the mechanical equivalent to using a transformer to trade voltage for current.

[Al_The_Man]

And as torque increases at the ratio rate of reduction, the motor to load inertia ratio decreases by the Square of the reduction.
Al.

[louieatienza]

Also in the case of dual leadscrew design to move the gantry, so moving the leadscrew out from under the work table...

[NEATman]

Say you have one machine with direct drive (motor on the screw) and the screw lead is 0.200".
On another machine you have a 2.5:1 belt reduction and the screw lead is 0.500".
You have the same resolution (motor rotation vs. distance) with both set-ups, but you can have a higher speed with the belt driven axis due to lower screw RPM, which is a big advantage for small diameter screws spanning a long distance, as it minimizes the potential for whip (the screw turning into a "jump-rope"). Also, if these are acme screws, it lowers the Surface Feet per Minuite within the nut, which can cause "squeal" or excess wear.

Keith

[Stuey]

Ah, I think I understand the gist of it a bit better now, thank you all for the clarification!

[MadMax]

Belt drive also has the benefit of folding an axis drive under the end of the screw. In some designs this is a handy way to reduce total drive length by tucking the motor length under the ballscrew. A belt also acts as a high torque capacity misalignment coupling. Misalignment couplings are a necessary feature to prevent a "three bearing in a row" design problem when driving a screw with a motor. Basically a motor has at least two radial bearings. A ballscrew would be supported by two or more radial bearings at the ends.

Joining a motor in line with a ballscrew would require that all four bearings (in motor and screw subassembly) be dead coaxial or you'd see some high radial loads due to misalignment as the bearings arm wrestled each other.

Dropping the motor out of plane and using a belt is a clean way to provide this secondary feature. Belts also can be quite economically strong. Misalignment couplings that have high torque capacity are comparatively quite expensive.

[TarHeelTom]

The very first Bridgeport CNC mill has the x & Y stepper motors tucked up underneath the table, with a belt drive for each.

Hate to thing how much access to the table would be impaired with the Y motor sticking out in the front and the x motor sticking out to the side.

Tom