Band Brake Torque Checks for Hoists and Holding Drums

A band brake is simple to picture: a flexible lined band wraps around part of a drum. One end of the band becomes the tight side, the other becomes the slack side, and the difference between those two pulls creates braking torque on the drum.

The simplicity is also the trap. A band brake can look strong because the band is wide and the drum is large, but the real capacity depends heavily on wrap angle, lining friction, direction of rotation, anchor geometry, allowable band tension, lining pressure, and heat. Treat it as a force-path problem first, not just a friction formula.

1. Start with the direction of rotation

For a band brake, direction matters. The same hardware can be self-energizing in one direction and much less helpful in the other. If the drum turns the other way, the tight and slack sides swap. That can change the required actuator force, the load on the anchor pin, and the pressure peak in the lining.

Before doing numbers, mark the intended drum rotation, the loaded band end, the actuated end, and the fixed anchor. Then ask whether the machine ever reverses. A holding brake on a hoist, a winch, or a test drum may see a preferred direction during normal service but can still be loaded backward during maintenance, rescue, lowering, or a fault. If reverse holding matters, check that case separately.

2. Use wrap angle as a design variable

The band tension ratio is governed by a capstan-style relation:

T1 / T2 = e^(mu theta)

Here mu is the effective coefficient of friction and theta is the wrap angle in radians. More wrap gives more tension ratio for the same lining and drum condition. But the gain is not free. More wrap may make the brake harder to assemble, increase sensitivity to lining wear, reduce cooling exposure, and make clearance control more awkward.

Use the coefficient of friction as an engineering estimate, not a guaranteed constant. Lining friction changes with temperature, moisture, oil, polish, wear, and surface finish. A brake that only works with an optimistic friction value is not robust.

3. Separate torque capacity from band strength

The torque from a simple band brake is:

M = (T1 - T2) r

where r is the drum radius. This equation says the useful torque comes from the tension difference, not from tight-side tension alone.

The band, pin, bracket, lever, and welds still have to carry the actual pulls. A common mistake is to calculate enough torque and then forget that the tight side may be several times the slack side. The tight-side end fitting, rivets, welded lug, or anchor pin can become the critical part. Design the band system around the largest force in the load path, then check the torque produced by the difference.

4. Check lining pressure, not only torque

For a first screen, the peak lining pressure can be estimated from the tight-side tension:

pmax = 2 T1 / (b D)

where b is band width and D is drum diameter. This is a useful quick check because the pressure is highest near the tight side. If the pressure is too high, the lining can glaze, wear rapidly, overheat locally, or crush before the brake reaches the expected torque.

Do not hide a pressure problem by adding more actuator force. If pressure is high, increase band width, increase drum diameter, use a lining with suitable pressure and temperature limits, change the brake type, or reduce the duty.

5. Heat can control the design

A holding brake and a stopping brake are different jobs. A holding brake may sit clamped with little sliding energy once the drum is stopped. A stopping brake converts kinetic and potential energy into heat at the lining and drum. The torque equation may pass while the thermal duty fails.

For a stop, estimate the energy that must be absorbed. Include rotating inertia, translated load energy, and any lowering or overhauling load. Then check whether the drum and lining can absorb and reject that energy at the expected cycle rate. Short occasional stops and repeated production stops are not the same brake problem.

6. Watch the actuator and anchor loads

The band ends do not disappear into the formula. They load pins, clevises, brackets, levers, frames, and welds. Draw a free-body diagram of the lever and anchor bracket. Check pin bearing, pin shear, bracket bending, weld group load, and bolt preload where relevant.

If the brake is self-energizing, the actuator force may look pleasantly small. That can be useful, but it also makes the brake sensitive to friction changes. If friction rises, the brake may grab. If friction falls, the required actuator force rises. Avoid layouts where a small change in lining condition creates a large change in field behavior.

7. Small worked example

A shop hoist uses a small band brake on a 300 mm drum. The band wraps 210 degrees around the drum, so:

theta = 210 x pi / 180 = 3.67 rad

The lining friction estimate for a dry, moderate-temperature condition is mu = 0.30. The band manufacturer and end connection are limited to a tight-side service tension of 1800 N. The band width is 40 mm.

The tension ratio is:

T1 / T2 = e^(0.30 x 3.67) = 3.0

If the tight side is limited to 1800 N, the corresponding slack side is about:

T2 = 1800 / 3.0 = 600 N

The torque capacity is then:

M = (1800 - 600) x 0.150 = 180 N m

Now check lining pressure:

pmax = 2 x 1800 / (0.040 x 0.300) = 300000 Pa = 0.30 MPa

This first pass says the brake can provide about 180 N m before the tight-side band limit is reached, with a peak lining pressure of about 0.30 MPa. If the required holding torque after service factor is 120 N m, the torque margin is 180 / 120 = 1.5. That may be acceptable for a lightly used holding brake if the lining pressure, pin loads, lever stress, and reverse direction check also pass.

Now change only the friction condition. Suppose dust, polish, or temperature reduces the effective friction to mu = 0.20. The tension ratio becomes:

T1 / T2 = e^(0.20 x 3.67) = 2.08

With the same tight-side limit of 1800 N, the slack side becomes 865 N, and the torque becomes:

M = (1800 - 865) x 0.150 = 140 N m

The brake still holds the 120 N m case, but the margin has fallen to about 1.17. That is a warning sign. The design may need more wrap, a wider band, a larger drum, a better lining choice, a higher-force actuator, or a different brake type.

8. Practical checklist

• Mark drum rotation and identify tight side and slack side.
• Check reverse rotation if the machine can back-drive or be serviced backward.
• Use theta in radians when applying the tension ratio.
• Use a conservative friction value for the actual lining, drum finish, temperature, and contamination risk.
• Limit tight-side tension by the band, lug, rivets, welds, and anchor hardware.
• Compute torque from the tension difference, not from tight-side tension alone.
• Check peak lining pressure and compare with lining guidance or test data.
• Estimate stop energy and cycle rate when the brake must dissipate heat.
• Draw the lever and anchor free bodies; check pins, brackets, bolts, and welds.
• Plan adjustment, wear allowance, guarding, inspection access, and safe release.

Bottom line

A band brake is attractive because a modest actuator force can create useful drum torque. That advantage comes from wrap and friction, so it is also where the risk lives. A sound first-pass check keeps four questions separate: does the brake make enough torque, can the band and hardware carry the pulls, is the lining pressure acceptable, and can the drum handle the heat duty?