Thread drive torque under load

In summary, the preliminary calculations for a basic lifting device were discussed, including motor data and output power. The motor is driving a 12-tooth gear meshed with a 72-tooth gear for a 6:1 rpm reduction, which results in an RPM of ~1385.16 on the threaded shaft and a torque of 0.55278 N-m. However, after applying the equation from the thread manufacturer for drive torque, the resulting torque is 2.5 N-m instead of the expected 0.55278 N-m. This discrepancy may be attributed to lubricant on the threads and the motor potentially putting out more torque than its rated amount. Further explanation and clarification were requested.
  • #1
Sentient
9
0
I am trying to do some preliminary calculations for a basic lifting device but some of my numbers seem to be out-of-wack. Could some kind soul take a look if you have a minute spare?

Motor Data (max efficiency)
RPM: 8311 = 870 radians/sec
Torque: 9.213N-cm = 0.09213N-m

Output Power = 870 * 0.09213 = 80.1531N-m/sec

The motor is driving a 12tooth gear meshed with a 72tooth gear for a 6:1 rpm reduction. The 72tooth gear is driving a 12mm trapezoidal thread (vertically mounted).

The RPM on the threaded shaft will be ~8311 / 6 = 1385.16RPM
The Torque will be 0.09213 * 6 = 0.55278N-m



The equation from the thread manufacturer for drive torque is:

T = (F * dm / 2) * (cos b * tan a + u) / (cos b - u * tan a) (kgf mm)

F is the thrust load (kgf) - I believe this is the weight on the nut that the thread is driving (for me, 160kg).
dm is the pitch circle diameter (12mm for my thread)
b is the Flank angle (given as 15 deg in the data sheet)
a is the lead angle (given as 3.31 deg in the data sheet)
u is the friction factor (I am using 0.20 as a generic steel-on-steel coeff)

Therefore:
T = (160 * 12 / 2) * (cos(15deg) * tan(3.31deg) + 0.2) / (cos(15deg) - 0.2 * tan(3.31deg)
T = 960 * (0.2558640807677) / (0.9543588758572)
T = 257.376 kgf-mm

converted to N-m

T = 257.376 * 9.81 * 1000
T = 2524858.56N-m

This number seems huge to me (2.5million Newtons/metre). Can anyone explain how this relates to the motor's output power. I know from practical testing that this motor can perform the task, therefore I think it's more likely that I don't understand the result from the second formula.

Could someone explain what the output from the second formula actually relates too?

Thanks
 
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  • #2
Hi Sentient
Sentient said:
I am trying to do some preliminary calculations for a basic lifting device but some of my numbers seem to be out-of-wack.

... converted to N-m

T = 257.376 * 9.81 * 1000
T = 2524858.56N-m

This number seems huge to me (2.5million Newtons/metre).
The reason it's huge is because you multiplied by 1000 instead of dividing by 1000. Should be 2.5 N-m. This is still quite a bit higher than the torque available to rotate this shaft (0.55278 N-m). I didn't go through all your math, but I'm assuming the rest was done right.

You say the motor is reliably turning this shaft, so the question must be how? Lubricant on the threads will reduce the coefficient of friction, and the motor may be putting out more than the rated torque, but it's pretty amazing that so much torque could be made up by those factors alone.
 

1. What is thread drive torque under load?

Thread drive torque under load refers to the amount of force or rotational energy that is required to turn a threaded drive mechanism, such as a screw or bolt, while it is supporting a load. This torque is important in determining the strength and stability of the threaded joint.

2. How is thread drive torque under load measured?

Thread drive torque under load is typically measured in units of force multiplied by distance, such as Newton-meters (Nm) or foot-pounds (ft-lb). This measurement can be obtained using a torque wrench or torque sensor.

3. What factors affect thread drive torque under load?

The thread pitch, diameter, and material of the threaded drive mechanism, as well as the type and amount of lubrication used, can all affect the amount of torque required to turn the mechanism under load. Additionally, the amount and direction of the load applied can also impact thread drive torque.

4. Why is it important to consider thread drive torque under load?

Thread drive torque under load is important because it directly affects the strength and integrity of the threaded joint. If the torque is too low, the joint may not be able to support the load, while if the torque is too high, it can lead to failure or damage of the joint.

5. How can thread drive torque under load be optimized?

To optimize thread drive torque under load, it is important to select the appropriate type and size of threaded drive mechanism, as well as the proper lubrication. Additionally, ensuring that the load is evenly distributed and properly aligned can help to optimize the torque required for the joint.

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