# Brake selection problem

I have a quick question guys. I am gonna select a brake but I am not sure.

A Crane Hoist motor has a rating of 300 HP and maximum speed of 1800 rpm. This motor is attached to a gearbox with a perfect (100% efficient) reduction ratio of 53.739:1. The API 7K specifications call for a safety factor of 2:1 (200% of motor full load torque) static holding torque from the brakes. The lining coefficient of friction is 0.40 (static) and the brake shoe face is 6.0 inches wide. Two (2) brakes need to be installed diametrically opposite each other on a large disc mounted directly to the cable drum (on the output side of the gear box).

Which disc diameter, and resultant brake torque is fine?

• There is a really strong smell of homework from this question. As this is not a free homework service so therefore you need to show your effort at solving it or the question will be closed. The question has nothing to do with regenerative braking or hydraulics. Why did you add those tags? Commented Nov 4, 2020 at 17:52
• Thanks for reply. :D, not really, that is a brake selection coming for a job interview. I put it here to keep the record for any future use! Commented Nov 4, 2020 at 18:23
• You need to clarify your thinking then if you're to get the job. Commented Nov 4, 2020 at 18:26
• I got the job. There not much online to solve such a problem. I sent my solution while ago and received second interview and job offer. That is a team leader position in brake industry. I do not deal with such problems. Commented Nov 4, 2020 at 18:46

The speed of the disk rotation is $$1800/53.739=33.495$$

The torque it creates is $$300*5252/1800=875.33 ft.lbf \quad 875.33*53.739= \\47040ft.lbf\rightarrow times 2=94079ft.lbf \ \text{at the disk}$$

If we assume the brake pad pressure $$P/inch^2$$ then its friction force $$F_f=0.4*P$$
$$\ 94079=2*F_f*6*R^2/2 \quad R^2=2*94079/2*F_f*6$$