# Ways to brake an axle

I am currently studying Mechanical Engineering and am doing an assignment where I have to design a wire rope tensioner where a wire rope is unspooled from a drum, but the wire has to be at a specific tension when unspooling. Therefore I am looking for ways to put an adjustable amount of resistance into the axle of this drum. I can't find much information on this subject.

Things I've thought of so far:

• A mechanism similar to drum brakes in cars (this is however not accurately adjustable and wears out quickly, also heat will form a problem).
• Using an electric motor for braking torque (Not sure if this is possible when the wire tension needs to be about 200kN, and what to do with the generated electricity).

I'm curious if you might have some more ideas?

• maybe something that runs the material over an idler pulley set up somewhat like a Derailleur controls tension? The tensioning force on the rotating arm with the idler could also be used as feedback to control a brake, but that's prob. unnecessary Feb 22 at 22:37
• Initially I read this as "break an axle" and started thinking of all the destructive ways to bend, twist, and shatter an axle shaft.... Feb 23 at 9:44
• The energy all ends up as heat. The brake disk heats up and the electrical brake generates current which you then dump into a resistor and it ends up as heat. The eddy current brake combines the generator with the resistor and it gets hot too. A fan brake warms the air. (or other media) Feb 23 at 22:35

In the wire & cable business, the device you describe is called a dancer, for the way its components move about while regulating the tension in a rope or wire being unspooled from a large coil.

The simplest examples use a rim brake which drags against the flange on the big spool. The rim brake is on a lever arm and the end of the arm is attached to a coil spring. That coil spring in turn is attached to a pulley through which the wire is routed.

Tugging on the end of the wire pulls the pulley, which pulls the spring, which pulls the lever, which pulls the brake shoe out of contact with the spool rim and allows the spool to turn and thereby feed wire off it. Letting go of the end of the wire releases the spring tension and allows the brake shoe to come back into contact with the spool, stopping it and thereby maintaining tension on the wire so it doesn't go slack and unwrap itself from the spool and create a tangle.

Spooling tensioners usually have a DC electric motor drive connected to the spool and a spring-and-pulley device which operates on the speed control knob for the DC motor. When you turn on the spooler, the motor comes on and begins rotating the spool which then pulls on the wire. as the tension in the wire climbs, the pulley shifts position, the spring deflects, and the tension control knob is turned down. When wire is then fed into the pulley, the spring tension is released and the tension control is turned up and the DC motor begins rotating the spool which takes up the wire.

The simplest upspooling tensioners have no mechanical control means at all; you simply adjust the current being fed to the upspool motor which sets its torque output at stall and you then feed in your wire. The motor runs until the tension starts going up in the wire, which causes the current flow up go up until the current limit is met at which point the motor stalls and the wire infeed halts at the desired tension.

One option that has no parts that wear out is eddy current brake. It consists of a metal disc, and a magnet in close proximity to it. The magnetic field will cause eddy currents in the metal disc, which in turn cause a torque that opposes the rotation. The energy is converted into heat in the wheel, which can be cooled down by either air flow or with liquid coolant.

Eddy current brakes can be easily made adjustable by using an electromagnet.

A downside is that the torque of eddy current brakes depends on the speed. As the disc velocity drops to zero, so does the braking torque - it can never bring the wheel to a standstill.

One way is to have a tensioning roller where the cable is held in contact or loops round it once. Looping round « locks » the cable to the roller - much like the positioning capstans used to move ships.

Then the roller’s speed can be controlled by a variable hydraulic pump to keep the correct tension . This way the friction, heat and adjustment can be controlled.

• This is the simplest solution for many tensioning applications. For example most Boeing aircraft (737, 747, ... 777 ) use hydraulic tensioning for braking the wheels. Mining applications use hydraulic tensioners on hoists. Etc etc Feb 26 at 17:48

Seen in a cinematic production years ago is a spooling system which used a series of paddles arranged on an axle. As the cable and human was pulled downward by gravity, the paddles spun in a manner similar to a fan. Drag increases by speed squared with a bit of a fudge factor, but it's sufficient to say that as the force of gravity accelerates the downward-facing human, the paddle drag increased to keep the speed to a reasonable level.

If your drum is large enough, the paddles could be stationary on the drum axle, which is filled with fluid. Cross-flow paddles mounted on the drum would provide turbulence and friction to the system.

I've seen similar devices in which the rotating component was spinning in a fluid, which has higher drag than air, allowing for a much smaller "paddle" assembly.

The advantage to these systems is a nearly-consistent release speed with minimal consideration to heating.

• sounds vagely similar to an erg machine, too Feb 22 at 22:43
• That was the other image I had in my mind, but it took your comment to knock it free. Feb 23 at 0:14

An automatic transmission and its convertor adjusted to output of proper torque, seems a good choice combined with a solenoid switch.

The output can easily be adjusted by a digital switch board.