Lessons · Engineering · Gear ratio trades speed for torque
A gearbox buys torque with speed
A gear ratio multiplies torque and divides speed by the same number, or the reverse. It creates no power -- the product stays the same, less whatever is lost to friction. A 5:1 reduction gives five times the torque at a fifth of the speed.
Hone is a place to practise a career, one idea a day. This is one of its lessons, written out in full and free to read without an account.
In beta. This lesson was written for Hone and has not yet been checked by a licensed engineer. Practice material, not professional advice. What that means.
What it is for
Because it is the cheapest fix in mechanical engineering and the most often forgotten. The machine that will not start under load usually needs a ratio, not a bigger motor.
How to think about it
Decide which quantity you are short of and which you can spare, then apply the ratio to both in opposite directions. Check by multiplying: torque times speed should come out roughly where it started, because power is conserved.
Worked example
Reduction ratio 5:1.Output turns five times slower.
Output torque = input torque x 5.Torque goes up by the ratio.
Output speed = input speed / 5.Speed goes down by the same ratio.
Power is unchanged apart from losses.Multiply the two back together to check.
Your turn
A 4:1 reduction is fed 15 N·m. Write the output torque in N·m, ignoring losses.
15 x 4 = N·m
Solve one, graded on the server
The trap
Expecting more power out. A ratio redistributes torque and speed; anything claiming to add power has a mistake in it.