Ballscrew design and 12vdc rpm and torque calculations help required

[John McNamara]

Hi RJSutton

I have been thinking on your project and considering a timing belt driven linear motion instead of a ball screw drive, this will simplify construction, be more compact and be cleaner.

Ballscrews need to be lubricated externally require bulky end support bearings and very accurate alignment between the ball nut attached to the moving member and the screw itself. Ball screws are not always self locking due to the low friction coefficient.

A timing belt drive is run dry, It can work successfully with some misalignment and can use simple off the peg sealed bearing units bolted on to or using flanged or circlip bearings clamped into laser cut frame members. For a project like this laser cut sheet parts should be a primary objective, remember they can be press brake folded after cutting.

You will be surprised at the load carrying capacity of a timing belt, the backing contains multiple strands of fine steel or kevlar.
Timing belt drives are extensively used in industrial robotic applications.
For reciprocating drives the belt can be purchased by cut to length, The two ends are attached to the moving member. Using a timing belt you would use a higher ratio worm reducer to achieve the same output speed.
Beyond a certain ratio worm reducers are self locking an important consideration for a lift drive subjected to gravity.

Below is a link to Gates among the world leaders in this field. Have a browse around the website you will find a lot of useful information..

https://www.gates.com/us/en/resource...x-pro-software

Regards
John

[rjsutton]

cheers John
Thanks for the input
Unfortunately this time im going to try the screw drive as ive purchased all the items already and waiting for delivery
the ballscrew is only used to move the platform horizontally . you mention "subject to gravity" which makes me think you have a vertical usage.
thanks Richard

[John McNamara]

Hi RJ

I assume the wheel chair has to slide out of the cargo space of the vehicle then be lowered down to the ground, in a way somewhat similar to the way a forklift works. In my head I imagined the chair parking on a plate that has channels for the wheels to locate them. When lowered to the ground the chair would be driven on to the plate and locked in place probably by clamping the wheels in some way. Once locked on, the plate would rise then slide into the cargo space with sufficient room for the cargo door to be closed.

Regards
John

[rjsutton]

Hi John
you are nearly correct
the "chair is actually lifted by the 2 grab handles ( these are bolted to the substructure of the chair and when they use a hoist the bracket hooks these) i have 2 fold out arms off the fork that have channels made in them that go under the round grab rails. A simple sliding plate then slips down to "lock" the handles to the machine.
this is lifted bu the fork plate and then it moves back into the car . As soon as the chair is back about 200mm the wheels actually roll on the sliding plate so the weight is really off the arms then. the whole plate -forklift etc rolls back inside a separate subframe. The reason being the main plate needs to move out of the car first ( it goes out 400mm with a linear actuator mounted below) so it clears bumpers-towbars etc
then the chair is moved out the full 900mm and lowered to ground. This way its only the weight of the chair thats lifted vertically not the plate etc as well
also you can park the chair first as the arms come out and down to pick it up. If it was on a plate then that would have to be in place first obviously .
I think its bloody ingenious ( but i would wouldn't I ) :)
the complete size is 1000 wide x 900 deep x 740 high which will slip between most suv type vehicles wheel arches and fit in the boot height. I drive a new CRV and it fits in there ( or will) when finished.The fork lift caused problems as ive reversed the chain-sprocket arrangement to only need 325mm to lift the fork-chair 650mm so it doubles the force required to lift it.tried 2x 4kn actuators which failed miserably . changed to a direct chain and sprocket drive thru a reduction box which could lift the moon. anyway im waffling
thanks for your interest
Richard