Motors

[ostopher]

Hi Guys,

I need to find a motor that "locks up" when it does not have power going to it. Do these exist and what would they be called?

 

[davidbeach]

You need to drive your load through a worm gear.

 

[thewellguy]

A NRR motor (non-reverse ratchet) is "locked" so that it will only go in one direction. More info would make your questions easier to answer.

 

[ostopher]

we're rotating a satellite dish which will occaissionally be under wind loading creating a torque of between 1 and 1.5knm. i'm looking at gearing solutions now but wondered if i could help by having a motor that would also provide a resistance to the windforce when it is powered down. and NNR isnt really applicatble as it needs to be able to turn clockwise and anti-clockwise.

 

[Clyde38]

If you are using a geared motor, you might look at "Power Off Brakes". They are mounted to the motor and provide static holding in the absence of power. I don't know of any mfg. that provides 1 to 1.5 kNm, though Ogura has brakes to 800 nM. There is also Deltran, Inetia Dynamics and others.

 

[ScottyUK]

David got it with the first post. Perfect application for a worm and wheel.


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If we learn from our mistakes I'm getting a great education!

 

[jraef]

They are called "brake motors" and have an integral spring loaded friction brake attached to the back (non-drive) end. The brake is help open by the same power going to the motor, so when you energize the motor the brake is released, when you de-energize the motor the brake is set. These are very common.

But as others have mentioned, a worm gear usually doesn't need one and on a satellite dish I have to imagine you have a very high gear ratio. Worth considering.

 

[waross]

I would not use a motor brake on a worm drive satelite positioner to hold position, the worm drive will do that.
I may consider a brake to provide faster stops for more precise positioning.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[jraef]

Sorry, I wasn't implying using a brake motor on a worm drive. Just the opposite; worm drive instead of a brake motor, since gearing is already necessary. I'm doing a solar reflector positioning system right now and we are using worm drives for that very reason. The MEs on my project had originally designed in a planetary gear and brake motor, I pointed out that I though the brake would cause the reflector to shudder when stopped. The worm drive with a VFD is smoooooth. We didn't even need a DB resistor on the VFD.

I only mentioned the brake motors because he asked if there was such a thing.

 

[jraef]

And for those of you curious sorts who will question why I used a VFD on a worm drive, I'll have to say that if I explained my complicated reasoning it would divulge too much about the project and I'd have to have you all killed to avoid violating my non-disclosure agreement.

 

[DickDV]

One caution about worm gears. They only lock up at ratios above about 28/1. At small ratios, they will back drive in both directions.

 

[itsmoked]

That's why one backed up on me.. I knew there was some sort of catch.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[golyos]

Dickdv youre right these worm gear locks up on specific ratio,i prefer brake motor for your rotating disk

 

[rsteves]

I believe there is no set gear ratio below which a worm drive will backdrive. The "backdriveability" depends on the worm diameter and pitch. It is only loosely related to the gear ratio since higher ratios tend to use finer pitch worms. Just as with leadscrews, the coarser the pitch and the smaller the diameter, the more likely a worm is to backdrive.

Rob Steves - Zaber Technologies Inc -

http://www.zaber.com

 

[roydm]

Guys,
I know this treads getting on a bit but here's my cents worth. I used to work on a lot of gantry cranes with Demag brand brake motors, these are quite interesting, the rotor is tapered. When energized the taper causes the rotor to suck into the stator. When de-energized a spring pushes the rotor out of the stator against a tapered brake shoe mounted where the fan housing would normally be. This seems simpler and required less maintenance than others where a separate solenoid activated brake is provided. I have never seen one operated by VFD but I imagine they would be OK.

Roy

 

[krwagner]

Actually there is a relationship to gear ratio and back driving capability with worm gearing. At higher single stage ratios the helix angle of the worm will be lessthan the stationary friction angle (angle created from the arctan of the coefficient of friction between the mating materials at rest). This condition is considered self locking aka won't back drive. However the friction angle changes with speed so introduce some vibration and enough force from the output and the gear set can back drive. The higher the ratio the lower the helix and less likely to back drive.

 

[Compositepro]

Why would you not use a commercial off-the-shelf antenna rotator? they are relatively cheap and have position feedback indicators. This is a very common application need. All the ones I've seen do not use worm gears. The gear ratios are so high that normal spur gears will not back drive. But if the antenna does move the position feedback will operate the motor to move it back.