This question may be too general, so apologies in advance if that is the case. I'm looking for a (relatively simple) mechanical system that will cause the driven element to move fast under no-load conditions, until it encounters opposing torque, at which point the system will automatically "gear down" to decrease speed but increase torque. Is there a name for such a system, or generally accepted solutions?

To make this more concrete, think of something like a vise. It's driven by a screw, so normally it trades input speed for mechanical advantage. But what if you wanted the jaws to close quickly (high speed, low torque), then "shift gears" as soon as resistance is encountered, to a low-speed, high-torque arrangement? You don't know in advance how big or small the item you want to clamp is, so you can't shift based on absolute position.

Obviously something like a car's automatic transmission can do this, but I'm looking for a simpler mechanical solution. I've been toying around with ideas about swinging/sliding gear carriers to bring elements into and out of engagement, but if there's a specific term I should be looking for or examples of something like this, I'd appreciate being pointed in the right direction!

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    $\begingroup$ this is the way mechanical CVT's function. $\endgroup$
    – Tiger Guy
    May 22, 2023 at 18:35
  • $\begingroup$ This is the way lots of vices function :) Quick slide to close, lock and rotate to grip. $\endgroup$
    – david
    May 23, 2023 at 3:42
  • $\begingroup$ @david I'd thought of something like that, but are there any that do it "automatically", or with a single motion? Every one I've seen like that requires you to disengage the jaw from the gearing, slide it, then re-engage it. $\endgroup$
    – stangdon
    May 23, 2023 at 17:07

1 Answer 1


As Tiger Guy suggests, CVTs are configured to perform in the manner you describe, primarily to provide variable ratios in a transmission for various transportation devices.

According to snowgoer.com, the first use of a CVT type transmission was 1950 and it has been improved as time has passed. Snowmobiles are not known for being designed for efficiency and the friction losses from using rubber belts to transmit power are considered an acceptable design compromise.

In automobiles, such friction losses are not as acceptable and designers have created a transmission that uses a rubber belt surrounded with metal links that are pushed by the pulleys rather than being dragged along the face of the pulleys as is the case with the rubber belt design. Note: this information was imparted in a Car & Driver article many years ago and I am unable to locate a current reference. The current listing from this magazine describes generally the operation but does not reference the efficiency or development aspect of the metal link belt.

If your application involves speeds as slow as the vise handle as described, you may have rather large levers internal (or external) to the drive pulley, as it is the speed of the rotation that provides the necessary force to change the pulley wall placement and therefore the diameter. Engines spinning at hundreds to thousands of rpm will provide plenty of force on small levers, while vise handle rates of rotation will require longer arms.

A CVT-based design will work exactly as you describe, providing fast rate (high gearing) when the output rpm is initially high (input rpm low) and will automatically gear down when the reverse is true.

Driving and Life has a simplistic diagram of the basics of the principle:

CVT diagram

Most of the automobile (and snowmobile) designs are configured in this manner. In my search of supporting material, I crossed paths with a peculiarly unique design for which I can find no proper link (motorauthority.com is the source of the image) and the image is included for the sake of completeness:

unusual CVT design

This particular design isn't as compact and appears to be more mechanically complex and possibly mechanically fragile.

  • $\begingroup$ Interesting - I'd run across mentions of CVTs (and even used one on rental bikes) and I certainly see how it could give you quite a range of ratios, but are there any that are "automatic" or "self-adjusting"? $\endgroup$
    – stangdon
    May 23, 2023 at 17:12
  • $\begingroup$ Without complex control mechanisms, they are fully automatic, such as those in snowmobiles. $\endgroup$
    – fred_dot_u
    May 23, 2023 at 19:25

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