Electric motors and gears: torque, rpm questions

• cgibsong002
In summary, the discussion revolved around finding the right combination of motor torque, RPM, and gear ratio to successfully operate a small electric motor attached to a gearbox that opens and closes a metal valve. Through research and experimentation, it was determined that a high gear ratio and low torque motor would achieve the desired outcome. Questions were raised about whether the same output torque can be achieved with different combinations of motor torque and gear ratio and whether the system would require different levels of force to turn the valve when the motor is disengaged. Suggestions were given to use a worm drive for a system that cannot be driven backwards when the motor is switched off, and to use limit switches to prevent damage to the valve.
cgibsong002
Hi guys, new here to the forum. I was an electrical engineering student but am now working on more of a mechanical issue so I'm a bit out of my element.

I am working with a small electric motor attached to a gearbox. The final gear opens and closes a metal valve. I am having trouble finding a combination of motor torque/rpm and gear ratio that successfully accomplishes what we need. Too much torque and the valve is damaged, too little and the valve gets pushed back open by air pressure.

Through my research and experimenting, it seems that a high gear ratio, with a low torque motor should accomplish what I need. It is my thought that, the high gear ratio requires more reverse force to push the valve back open from air alone. Through testing the higher gear sets are way more difficult to turn the valve (but is this due to gear ratio alone or also resistance on the motor gear?). But a low torqued motor should still be able to turn the gears and will not over torque the valve.

For reference, the gear sets I have tested so far are between the ratios of 400 and 1100. I am hopefully calculating these right. Each gear set has either 4 or 5 compound gears (so 8-10 in total). I am working on getting motor specs, but I am unsure if there's any way I can actually calculate torque on the motor.

Does final gear RPM*gear ratio determine motor rpm? Because final gear is very slow and easy to observe/measure.

Thanks for the help! I apologize if I'm a bit off on my thinking.

Motor RPM / Gear ratio = Output RPM

Motor Torque * Gear Ratio = Output Torque

edit:next post.

Last edited:
Ok, so I finally actually dug up actual specs on the gear set and motors. My calculated gear ratios were correct.

So, motor torque*ratio=output torque. According to this, it doesn't matter whether my power is coming from the motor or from a higher gear ratio, as long as the one is raised/lowered to give the same output torque.

My question is.. does this hold true in reverse? As in:

motor torque (20g*cm)*gear ratio (1000) = 20,000 output
motor torque (5) *gear ratio (4000) = 20,000 output

In one case the motor is strong with a weak ratio, the second has a weak motor with strong gearbox. In either case, when power is applied to the system, you have the same output torque. But what about when no electrical power is applied? If simply force is applied to the output (in this case our valve), will it take the same amount of force to turn each one, or now that the electrical motor is disengaged will it require more force to turn the unit with a larger gear ratio?

If you want to turn the output, you must supply some input torque, whether it comes from an electric motor, your hand, or a diesel engine, etc.

But what about when no electrical power is applied? If simply force is applied to the output (in this case our valve), will it take the same amount of force to turn each one, or now that the electrical motor is disengaged will it require more force to turn the unit with a larger gear ratio?

If you want a system that can't be driven backwards when the motor is switched off look at a worm drive.

I would use limit switches to stop the valve being damaged by the motor.

1. What is torque and why is it important in electric motors?

Torque is the measure of a force that causes an object to rotate or move around an axis. In an electric motor, torque is the force that enables the motor to turn and perform work. It is important because the amount of torque produced by a motor determines how much weight or load it can move, and ultimately affects the motor's output and efficiency.

2. How is torque related to RPM in electric motors?

Torque and RPM (revolutions per minute) have an inverse relationship in electric motors. This means that as torque increases, RPM decreases, and vice versa. This relationship is important to consider when selecting a motor for a specific application, as it determines the motor's speed and power output.

3. What factors affect the torque output of an electric motor?

The torque output of an electric motor is affected by several factors, including the number of poles in the motor, the strength of the magnetic field, and the amount of current flowing through the motor's windings. The design and construction of the motor also play a role in its torque output.

4. Can gears be used to increase or decrease torque in an electric motor?

Yes, gears can be used to increase or decrease the torque output of an electric motor. Gears work by transferring torque from one gear to another, either increasing or decreasing it depending on the gear ratio. This allows for more precise control over the motor's output and enables it to perform different tasks with varying levels of torque.

5. How do I calculate the torque and RPM needed for my specific application?

Calculating the torque and RPM needed for a specific application involves considering the weight or load being moved, the desired speed and power output, and the gear ratio (if gears are being used). These factors can then be used to determine the necessary torque and RPM specifications for the motor needed to perform the task effectively.

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