# DC motor idle holding torque?

I’m looking to be able to raise, hold, and lower something weighing approximately a kilogram, so a torque of about 25 mNm (3.5 oz in). This is well within the stall torque of something like the 150:1 Micro Metal Gearmotor, but will it be able to hold that weight for long periods of time without using any power?

As an extra question, is a motor that small suitable for such a thing? Space is rather limited and I would rather not go for one of the 25Ds or bigger.

Hello.

If you want something that you cannot back drive, you should consider a gearbox that includes a worm gear. We sell a couple of Tamiya kits that have a worm gear:

I am not sure if they can stand up to your application, but you could buy one and try for yourself.

- Ryan

First: I’m not sure your calculation is correct. 3.5 ounces is only 1/10th of a kilogram, so your maximum radius is only 1/10th of an inch – 2.5 mm. That’s not a whole lot of distance for “raising and lowering.” I’m assuming you’re building a winch?
There are 10 N to 1 kg, perhaps that’s where the error comes from?

So, let’s say you have 35 ozin of stall torque, which means 2.5 kgcm. This means that, if you have a horn/winch/whatever that has a radius of less than 25 mm, the motor can turn even when counter-loaded by your one kilogram of load. If you use a horn of 25 mm radius, this limits your total range of movement to less than 50 mm. If you use a winch, it may work, as long as the total radius of the accumulated wire is less than 25 mm.

Second: The motor will not be able to hold the raised load without using any current. If you remove current, you remove force, and whatever you raised will come falling back down. You need some separate locking mechanism. This can be a ratchet, a locking pin, or some other mechanism, probably actuated either by a solenoid, or some mechanical lock at the end of the range of motion.

The calculations are correct - a 5mm shaft at ~100rpm can raise a load at about 2.5cm/s, which is sufficient for my needs. We’re only talking a distance of 30-40cm so accumulation of the wire (fishing line) shouldn’t be an issue.

I’ve an old motor + planetary gearbox that has a built-in passive locking mechanism but it’s a little large and I can’t find anything similar that’s suitable/cheap. I’ve looked at gearboxes with worm gears but none seem quite right. I was hoping what little resistance an unpowered DC motor has, with the gear reductions and additional friction, would be enough, but I guess not. This is a one-off build so I don’t want to be buying motors just for testing.

Thanks anyway.

- Tim

Hi, Tim.

I just took a 150 HP and non-HP micro metal gearmotor out of stock and measured the torque it took to turn the output shaft with nothing connected to the motor leads. Both versions behaved similarly, and it seems like the torque required to start it turning was typically between 4 and 8 oz-in (there might be more variation from gearbox to gearbox, and it also seems to vary with output shaft position). Once the shaft started turning, the required torque to keep it turning dropped significantly. In general, the higher the gear ratio, the greater the torque it will be able to withstand while unpowered. If your estimate of 3.5 oz-in is correct, the micro metal gearmotors should be able to do what you want if you get a high enough gear ratio. For example, if you go with a 298:1 HP, you can get around 100 RPM and the gearbox should be able to withstand quite a bit more than 3.5 oz-in.

- Ben

Thanks Ben. Am I correct in thinking that with the motor in brake mode (the two leads connected together, but still unpowered) it would be able to withstand even more than this? Or perhaps it would just increase braking torque while already turning.

I expect the magnitude of the counter-torque due to the braking effect to be proportional to the motor speed, so I don’t think that would do much to help hold a load stationary, though maybe it could effectively help decrease the disparity between the gearbox’s static and dynamic friction. If you have the ability to trivially connect the two motor leads together in your application after cutting power, I suggest you try it, if only to help slow the rate of descent should the motor not hold properly.

- Ben