# How do I add a resistance to mechanical movement in both directions, apart from friction?

What options do I have for creating a resistance to a mechanical movement apart from simple friction?

Assume I have a lever, which will result in either circular or linear motion. I want to be able to apply a resistance to this motion, so that the user will have to apply extra effort to move the lever in either direction. I could use a spring, which would resist motion in one direction, but this would then result in a restoring force when the initial displacement force was removed. I want the extra force to act in both directions, so that the lever is "heavy" to move, but will stay in its new position when displaced.

I want the force to be able to be calibrated and remain constant, so I don't want to have something like a brake pad, which will wear, or just a nut to do the joint up tight, as this will be too difficult to achieve a calibrated force.

Is there something that could be done with a spring or pneumatic air muscle or by using hydraulics?

• magnets :-). No, seriously, a solenoid-like setup would work, but is unlikely to be a practical solution for you. Jan 17 '16 at 16:47
• How would you envisage using magnets please?
– John
Jan 17 '16 at 23:11
• It's partly a joke (see Insane Clown Posse), but in fact a short rod of iron placed in the axis of a cylindrical magnet -- which is easier to make if you wind a solenoid coil -- will be restrained by the magnetic field to stay more or less centered, so it'll resist being pushed/pulled in either direction. Jan 18 '16 at 13:02
• Use the lever to propel a flywheel through a set of gears, for a particularly satisfactory tactile feedback. Once put in motion, it continues to move and you must apply force to stop it!
– SF.
Jun 8 '18 at 12:09
• If I'm understanding your Q, consider using a mechanical detent of some sort. See: en.wikipedia.org/wiki/Detent Or more specifically a en.wikipedia.org/wiki/Ball_detent Jun 8 '18 at 13:17

The two basic mechanical options are either a viscous damper or inertia or indeed a combination or both.

Of the two a damper is probably the closest to what you want as it will provide a force which opposes the direction of movement which is proportional to the speed of movement.

Adding mass (perhaps as a counterweight and pulley system) won't on it's own do exactly what you describe as it resists acceleration but it may be that a small adjustable mass will help you to fine tune the feel. The downside of a mass damper system on its own is that it won't provide a positive 'lock' and the position could creep under accidental pressure or vibration so it's more useful if the lever is for constant adjustment rather than being set to one position and left.

There are quite a lot of off the shelf dampers available in liner and rotary formats and many are adjustable. You also have the option of using cams or bar linkages to have variable or progressive resistance across the travel of the lever.

A more sophisticated option is to use an active system with something like a solenoid or pneumatic actuator which provides active force feedback and even maintains an exact position below a minimum force input. In this case you could design an electric control system to provide whatever force feedback profile you wanted.

Having said that, a friction brake is the simplest way to achieve exactly what you want as it provides a constant resistive force. To be honest, the wear rate of the friction material in this context is not likely to be significant, especially considering that hydraulic and pneumatic systems require a certain amount of maintenance in themselves.

• A viscous damper pretty much is a form of friction. Jan 17 '16 at 16:47
• True but you could argue that anything which answers the problem (ie a proportional resistive force opposing movement) is some form of friction. Jan 18 '16 at 18:58
• I don't think I agree that a repulsive magnetic field is friction. Jan 18 '16 at 19:23

Elaborating on CarlWitthoft's comment, magnetic damping, sometimes called eddy-current damping, is a standard way of introducing damping in metallic systems. The principle is fairly simple; as a magnet moves past a conducting material like a metal, it induces circular currents (eddy currents) in the material. The resistivity of the conductor converts this current flow into heat, taking energy out of the system.

The most dramatic example of this is dropping a strong magnet down a copper tube (cool videos here, here, and here).

• Thanks very much, that's a remarkable series of videos: I've not seen that effect before. I take it that the requirement is for a non-ferrous sleeve? Also, do you know what the relationship is for the braking force?
– John
Jan 18 '16 at 9:24
• @John A rigorous derivation of the relationship can be found in these lecture notes from a Princeton professor. It is a velocity dependent relationship that is proportional to the velocity squared: $F\propto v(t)^2$. Jan 18 '16 at 12:54

Dampers provides resistive force as a function of velocity, while friction is more constant. You can buy an off the shelf damper: look at the compression and extension section in the following link for dampers that work in both directions: http://www.mcmaster.com/#adjustable-dampers/=10q42q4 If you are looking for something less expensive, get an used pneumatic cylinder with ports on both ends, and run a small tube from the port on one end to the other port. If you need more damping, add a manual flow control to the tube, or fill the cylinder with oil.

Your question immediately brings dampers into mind, but I think you might still be happier with friction. It depends on whether you want the resistance to increase with speed of movement (damper) or to remain constant (friction).

To keep the friction steady over the lifespan of the device, you can use a spring to push a brake pad against the rotating surface. If they are both made of e.g. steel, wear should be quite slow. As the surface wears, the spring will keep the force constant even if the dimensions change a millimeter or two.