A self-blocking gearbox (with worm screw) can be driven only from the screw.
The worm cannot be back driven by the associated gear because of the thread pitch of the worm. It locks in place when the worm is not moving.
Ok but can it go in two directions? Both lift up and down? In that case how did you do that? I have been thinking of a solution like that but the only way I can come up with is to use electromagnetic brake if I want it to go in two directions. But a mechanical solution might be both better and cheaper ...
Sorry didnt quite understand. Only from the screw? Are there selfblocking gearboxes that somehow can change directions?
A worm and gear arrangement can only be driven from the worm, and the worm can be driven in both directions. So it sounds like it meets your needs. Remember - this arrangement has a large reduction ratio.
This is basic mechanics - it might be useful for you to do a bit of research and learning around the subject. There are lots of good vids on YouTube.
I'm going to repeat my original question: why does it have to be a stepper motor?
Selfblocking is independent of direction, unlike a ratchet.
no it doesnt have to be a stepper
Ok sounds like a solution. And a wormdrive solution is always selfblocking right?
Some applications need to have the worm back driven by the gear. In that case the pitch of the teeth on the worm are designed with a very high angular pitch so it can be back driven. But very rare. Can't make such a universal statement.
Basically, yes. As @Paul_KD7HB says, if the pitch is steep enough then it can be back-driven. The only example I can think of is the old telephone dial mechanism, where a back-driven worm was used to drive the centrifugal governor:
HOW DOES A BRITISH DIAL WORK
But this is rare. Basically you can safely assume that a normal worm drive cannot be back-driven.
I did some more research and found these selflocking dc motors with worm gearbox that seems promising (and cheap) with the specs:
No Load Speed(RPM): 30
Rated torque(kg.cm): 10.0 Kg.cm |
stall Torque: ≥ 32 Kg.cm
If I understand the specs right and calculate it correct it should be able to hold about 10 kg (3.1 Nm or 32kg.cm) and pull a load of about 3 kg in both directions? Im not sure about the last calculation though. Maybe it is less though with an diameter of a wheel or arm of 4-6 cm?
There is also this one that is even cheaper but cannot hold that much.
this should be alright right to pull my 2-3 kg load up and down and lock it when it is not in use or when the power is off?
I say - block & tackle, anyone?
Yes, a blacksmith forge blower!
Firstly is a stepper actually needed? Could you simply use limit switches?
I know this is late in the game, but I think the O.P. shows a confusion about torque:
Torque is force times radius, not the distance of the linear motion.
You need to convert rotation to linear motion to use a motor to lift something, either with a lead-screw or gears or pulley system.
For instance if you used a drum of radius 2cm to wind up the load (20N) on a thread/wire, a torque of 0.02 x 20 = 0.4Nm would be the minimum needed.
You might have gearing as well, 10-fold reduction multiples motor torque by 10 (in theory, but there will be losses).
A leadscrew with pitch d (in metres) will convert a linear load of F newtons to a torque of:
d x F / (2 x pi)
So if the pitch is one turn per 2mm, and the load is 20N, that's about 0.002 x 20 / (2 x pi)
= 0.0065 Nm (neglecting friction).
This worm gearbox is self-locking, and the article above also has a brief description of worm gearboxes.