Friction Brakes Ergometer: Novel Application

[dalvares]

Dear All.

I am working on a uni project building an ergometer for an MRI machine. I was interested in purchasing a disk brake system to control the resistance and the power output for the ergometer. The application requires that all parts to be non magnetic as it will be used in an MRI machine (Aluminium and Stainless Steel ok). Because of the hydraulic brakes for mountain bikes are readily available I was hoping to purchase one and just replace the bolts etc. I understand given the usual application of these systems that it unlikely that the brakes conform.

I was given feedback that the system might overheat and fail as they are not design to be dragged constantly. I understand this is probably true in a mountain bike however the pedal will operate at a max of 90rpm and subjects will dissipate only 100W of power. I would still like to calculate the heat dissipation and the temperature change in the pads and disk.

So far I know the power so I can calculate the work done by the subject when the brakes are fully depressed over the duration of the experiment. Assuming all is dissipated as heat I can then use the formula

Qm=m*c*T
mass=m
specific heat cap=c
Temp=t

Assuming that I am on the right track so far, I was lost when I tried to figure out how the heat would be dissipated. There's 2 brakes pads (Caliper brakes) 25mm*25mm and the rotor is 160mm OD. (Assuming its a flat disk with no ventillation). Is it half on the brake pads and the other half on the rotor. Also where does the coefficient of friction come in or is that not really important for this aspect.

I have simulink MATLAB at my disposal. If you think I might be able to do a more accurate job on that can you guide as to how I would set up a model and what equations I would use.

Really appreciate any help I could get.

Looking forward to hearing from you soon.

Cheers
Darren

 

[TVP45]

Darren,
Depending on your specific details, the aluminum might be a problem. It is non-magnetic but very conductive. If you are rotating it in a strong magnetic field, you'll get lots of eddy currents. You may have to look at materials with a much higher resistivity (titanium for example). The stainless is probably OK, but even that needs to be checked.

I saw a device something like an dynamometer for an MRI a couple years back at NIH. You might Google them. Look for a Dr. Balaban; I don't think he's the PI but I think he was involved.

 

[FredGarvin]

Friction brakes are never really the way to go for these types of applications for exactly the reason you stated; they are not designed for constant drag. This is where something like a water brake or air brake is really what needs to be used.

Along the lines of what TVP mentioned, be careful with the aluminum as well as the stainless. The 400 series is magnetic as well as others like the PH steels.