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Topic: Gears vs Belts (Read 3716 times) previous topic - next topic


I'm putting together a project to control my gas water heater by turning the knob on the control valve.
I measured the torque required to turn the knob (crudely by attaching vice grips to the knob, then measuring the weight to turn it). Conservatively it needs 5.75 kg-cm. I have a stepper motor rated for 2.3 kg-cm so I thought 1:2.5 gear ratio would be appropriate.
My question is whether I should use gears or a belt. I'll have to devise some way of attaching the motor assembly to the gas heater controller vale and thought that a belt would be a little more forgiving in what will likely be a difficult mounting. I was considering mounting the motor assembly with a light spring to keep tension on the belt.
Any comments or suggestions appreciated!


The knob on the control valve has maximum turn angle?


The knob on the control valve turns perhaps 300 degrees.


If you want to use a belt then it would need to be a toothed belt.  The advantage of a belt drive is that the motor could be offset some distance from the valve.  Spring mounting should not be necessary since a correctly tensioned tooth belt can be "rigidly" fixed.

However, if you can mount the motor intimately with the valve spindle then i'd recommend a simple gear drive.  No slippage guaranteed !


In addition to timing belt / timing pulleys as jackrae described a second option would be roller chain. With plastic, 1/4" pitch roller chain you can easily adjust the length of the chain. A good vendor would be www.sdp-si.com for either method; you might be able to get by for under $20.


The knob on the control valve turns perhaps 300 degrees.

If you decide go though gear drive path,

An other way- relatively new is use smart stepper motors driver;-

Smart stepper motors driver (IC) has stall detection to detect
'home' position and keep track of forward and backward steps to know where you should be. Missed steps, or someone/something manually turning the shaft will break this method.

This will give you position switch less and encoder less solution.


5.75 kg-cm ?   You must have very strong hands.


suggest you meassure the angle of the knob directly,
    and 'just' move it to the 'correct' place.

that way, if the motor / chain / pulley / gears slip, you still know where the knob is.


or indirectly measure temperature, once it is in the stable stage.


Torque is gram-cm, not kg-cm.


Torque is gram-cm, not kg-cm.

So many misunderstandings of S.I. in one glib statement!

Firstly you don't mix names and symbols, so g-cm or gram-centimetre, but never gram-cm.

Secondly the kg is a unit of mass, not force.  The "kilogram-force" or kgf would be a suitable
force unit. In SI the kilogram is the base unit, not the gram (historical reasons).

Thirdly in S.I. the newton is always the prefered unit of force, and metre the prefered unit of
length, so newton-metre or N-m is the unit of torque.  And never N-cm (you can say cN-m though ;).

Now a newton is a joule/metre, so newton-metres are simply joules, but that's forgetting the
rotation, hence joules/radian is actually a more logical unit but for some reason N-m is used rather
than J/rad.

Anyway at least no-one's tried to use "kg/m" or "pounds/foot" or some such real howler :)
[ I will NOT respond to personal messages, I WILL delete them, use the forum please ]


Torque is gram-cm, not kg-cm.

lb-ft or oz-in all the way!!

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