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# motor torque question

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#### oem_odm

##### New Member
hello there

this question is about motor torque. please see the attached image and note the following.

A shaft is loaded with a weight of 5kg. One end of the shaft is mounted on a bearing, its other end connected directly to a motor. The motor is specified as having a torque of 1Kgf.cm

But what does that actually mean. Does it mean it can only turn a shaft loaded with 1KG?

I would like to understand better the relationship between 1Kgf.cm and what weight it can actuallly turn on a shaft.

many thanks

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1Kgf.cm is torque, not force. It means that with a lever arm of 1cm, a force of 1kg.f will develop at the tip. If the lever is double that, the force developed at the tip will be half that. If the lever is half that, the force developed at the tip will be double. THe lever arm might also be the radius of a wheel. IT doesn't have to be a beam.

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1kg/cm means that if you attach a rod perpendicular to the output shaft that at a distance of 1 centimeter it would be exerting 1kg of force. The way the mass is setup in your image has nothing to do with how torque is measured, the more weight in that case it would take longer for the shaft to get up to it's full rotation speed.

ok

so we're saying the kgcm rating has no influence on the motors ability to rotate the shaft its connected to.......bearing 5kg

im stuck then

what spec of motor would be needed to turn the shaft in my picture...?

Well, your drum isn't a lever arm since it's balanced all the way around which means that to make it spin you don't need to fight any forces like gravity. But it does take effort to accelerate it to get it spinning. It's similar to applying a force to an object floating in space (like sticking a rocket onto it) to make it accelerate faster and faster, except for a rotating system. The rocket thrust would be like the torque- the more thrust/torque you have the faster the object will accelerate/spin-up to max speed. The velocity of the rocket exhaust is like the no-load RPM of your motor, and sets the maximum speed. The mass of the object in space is like the first moment of inertia for your drum- it's a measure of how much mass is located away from the axis of rotation. Torque and the first moment of inertia for rotating systems are just like force and mass for translating systems.

Using a motor with very low torque but very high RPM is like using a rocket with very high velocity exhaust but very low thrust. It will accelerate very slow, but is capable of reach very high speeds given enough time to accelerate. Conversely, using a motor with low RPM but very high torque is like using a rocket with lots and lots of thrust but low exhaust velocity. You will accelerate very quickly but your top end speed is quite low.

Another example is a car that only has high gear or low gear. Low gear has lots of torque so you accelerate quickly but your top end speed is low. High gear has lots of speed but low torque so although you have accelerate very slowly, given enough time to accelerate, you will be able to obtain a speed that is faster than the car with low gear. But if you need to start and stop a lot, you don't have enough time to accelerate to your maximum speed which is when the low gear car might be better. This holds true your case too (and other moving vehicles). It's pointless to have an insane top speed if your acceleration is too slow since you will probably never have enough continuous acceleration time to ever reach that top speed.

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No absolutely not in fact I specifically stated that it would influence it.
the more weight in that case it would take longer for the shaft to get up to it's full rotation speed.
Think of an object at rest with the mass of the weight, and it's velocity as the cylinders rotational speed. Apply a fixed force to it and it will take X period of time to accelerate, given whatever losses are present. Assuming the motor could overcome the friction/drag loss, ANY motor could turn the weight, it would just take longer to get it up to a desired rotational speed. You haven't asked a complete question at this point.

easy way is to compare with newtons second law,
F=ma
F- force
m- mass
a- acceleration

in this case you have a torque-T
inertia- n (you shold know this quantity for the load connected to work out)
angular accelleratin w*

so it relates
T=nw* ( angular motion)
same as F=ma (linear motion)

so in reverse your motor delivers a torque and you should know the inertia (given data) its connected to then can find w*.
then you can say how long it may take for the mortor to come to full speed, then the motor doesnt deliver any torque in ideal situation (no losses) when at constant speed,

to find out the inertia only the weight of the load is not sufficient, the physical shape would influence on inertia. ie at how far every tiny particle of the load are located from the axis of the shaft. you need to intergrate it.

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so in a nut shell:

torque isn't really important becuase the weight isn't leveraged to the motor shaft. a bit of torque is needed of course to get it going.

..so i could loaded the shaft with 10k of weight, and, a 1kgcm 30rpm geared motor would turn it fine..?

thanks again....

when you try to pull it from zero RPM, there will be a resistive torque by the inertia, like when you try to pull some thing on a surface, your motor & gear assembly should be able to over come or should be able to deliver more torque than that resisting torque then of cause it will slowly accellerate and come to the final speed.

if your load is radially distributed ie larger diameter then you need more torue than that of distributed along with the shaft (lesser diameter)

so in a nut shell:

torque isn't really important becuase the weight isn't leveraged to the motor shaft. a bit of torque is needed of course to get it going.

..so i could loaded the shaft with 10k of weight, and, a 1kgcm 30rpm geared motor would turn it fine..?

thanks again....

Pretty much as long as the "weight" of the drum on the shaft was balanced. BUt let's not forget how too small a motor might burn out because it spends too much time at too low an RPM accelerating the drum, thereforing drawing too much current for too long a time.

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thankyou everybody very much.

i can see now that if the weight is evenly distributed radially over the shaft, the torque requirement should be low. What i need to be aware of only for torque is where radial weight distribution is uneven.

thank you all again

Hi there,

ANY motor will theoretically turn ANY weight perfectly centered on a shaft
eventually, even if it took an hour to get up to speed. The problem factor
is the sticking friction. If the sticking friction is too great than a smaller
motor may not be able to turn the shaft given any length of time. It's not
the inertial itself that has to be overcome, as that will always be possible,
but it's the sticking friction that actually has to be overcome to get things
turning. The inertia only slows the process down, but doesnt prevent it
from happening. It might also be interesting to note that a tiny motor
1/2 inch in diameter and 1 inch long could turn a balanced weight of several
thousand pounds if it were not for the sticking and sliding frictions.

Unfortunately, with many objects the sticking friction increases with weight.
Also, the bearing type and lubrication make a difference in the sticking friction.
These two, taken together, mean that for some weight with your given bearing
system the motor may not be able to turn the object even if it is perfectly centered
on axis.
If this happens you'll know it right away because the motor will grunt yet nothing turns.
To get around this, you either have to get better bearings or a stronger motor, or
perhaps a small 'push' type solenoid that can give the load a slight push to get
it started. Once started, the motor only has to overcome the sliding friction, and this is
often much less than the sticking friction so it might be able to keep it turning.
Again the inertia will slow down the process of speeding up however, so it may
take some time to get up to speed depending on weight.

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thanks Mr Al - very clear.

how would i configure this double pendulum setup to get as motion out of it as possible? The loaded shaft would be loaded with upto 5kg mounted on bearings on each end.

The pendulum shafts can be arond 30cm long, the pendulum weight could be several kg. Each time the pendulum rocks in a clockwise motion, its hib would pull round the loaded shaft. On its anti clockwise rock, the hub would slip (no turning the loaded shaft) - (only want loaded shaft to turn clockwise).

interested if this would be workable, say, for 15 minutes if the pendulum shaft is given a rock every 15 minutes....

can you put more exact sketch of your system with weight and all? also i couldnt get your full idea can you make your explanation so easy by telling where you are going to implement it? it will be helpfull to sugest you exactly.

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