Re: [CAD_CAM_EDM_DRO] Belt vs. Direct Drive

Dennis wrote:

> > Can anyone comment on the +/-'s of belt vs. direct drive
> > from a stepper to a ballscrew. I am concerned about
> > alignment of the coupling in direct mode vs. any extra
> > backlash in a belt drive mode.

Bob Campbell wrote:

> I use a flexible coupler between my stepper motor and my lead screw.
> The flexible coupler will take up a small amount of misalignment.

I too like using a flexible coupler on one end of your leadscrew
for routing applications (which on CCED are mostly homebuilt routers).

Over-constraint of the motion causes binding, and with a flexible
coupler, one end of the leadscrew is constrained (for tension and
compression) while the other is 'free' to move a few fractions of a mm,
relieving binding as the screw heats up, or the frame vibrates or
the original alignment was off, or whatever.

The axial load on the motor is nil, the radial load is not much (much
of the weight of the motor-end of the screw is actually supported by
the gantry), and so the motor is left to do what it does best: provide
torque.

A belt drive setup (great for increasing torque) radially loads the
motor bearings (still ok within limits), but increases the backlash
because the tensions of the timing belt is not equal on either side,
therefore on reversal the tensions will swap, and add to the backlash.

Specifically you are mentioning steppers. Steppers have greater torque
but lesser speed (compared to same-sized servos). Therefore it is
more likely to use a servo in a belt-drive reduction scenario because
servos have high rpm (which will be reduced by the belt drive), but
essentially low torque (which will be increased by the belt drive).

My (humble) opinion, is that if you are going with the expense of a
ballscrew on a router (to eliminate backlash), don't introduce some
more backlash by using pulleys (if you can help it), all other things
being equal (which they never are!).

Doug Fortune
http://www.cncKITS.com


















.