Packard Forum Index » General

DISCLAIMER: Use of this information is at your own risk and you assume all liability. No warranty is expressed or implied.

Introduction
I found this interesting and helpful thread on the AACA forums today:forums.aaca.org/topic/443226-brake-band-lining-suppliers/#findComment-2881392

For many of our cars, and of particular note the late-model senior cars, replacement brake shoes are no longer available. The post above gives a primer on some considerations when selecting a replacement lining. Though, it is mostly written from the perspective of choosing something for external band brakes, so some recommendations like choosing a "soft" lining are not as relevant.

For cars with mechanical (cable) brakes, see this comment:forums.aaca.org/topic/443226-brake-band-lining-suppliers/#findComment-2882670

Some favorite suppliers are McMaster or Scan-Pac. Whatever the lining, it is best to choose a non metallic type to reduce the wear rate of the mating component (i.e. brake drum); likewise, rivets must be brass or copper and not plated steel.

In particular, Green Gripper Aramid Uncured (Non-Metallic Semi-Flexible) seems to be a solid choice for many applications.

Choosing a Lining
When choosing a replacement lining, you must consider the maximum linear speed, maximum contact pressure, and coefficient of friction.

Maximum Linear Speed
The maximum linear speed is easily derived, where:
- R is wheel revolutions/mile (found in the service manual Specifications section, or derived using the tire circumference)
- d is the drum diameter in inches
- v is shoe velocity in ft./minute per car MPH

The resultant v can simply be multiplied by 100 to give the required speed for a car moving at 100 MPH. This gives a safety factor for worn or "incorrectly" sized (e.g radial) tires. (For pre-war cars, choose a more reasonable reference speed based on the engine redline).

For example, a 55 Clipper moves at 731 revolutions per mile and has 11" drums. This gives a minimum recommended speed of 3500 ft/min.




Maximum Contact Pressure
The maximum contact pressure may be estimated via the theoretical method described in Ch. 16 of Shigley's Mechanical Engineering Design, 11th. Ed.

Note: don't bother with your desktop calculator as I set up a custom calculator for these equations. Link is at the bottom of the article.

When simplifying the governing equations, where:
- f is the coefficient of friction
- p_a is the maximum contact pressure for the lining in PSI
- b is the lining width in inches
- r is 1/2 d (in inches)
- theta_a is the angle where the maximum contact pressure will occur; it is 90 degrees (2*pi radians) from vertical (assuming that the lining extends through that point, which they all would)
- theta_1 is the angle traced from the lower pivot to the lower end of the lining material, as the shoe is installed
- theta_2 is the angle traced from the lower pivot to the upper end of the lining material, as the shoe is installed
- a is the distance from the center of the lower pivot to the center of the axle, in inches
- c is the vertical distance from the center of the piston pin to the center of the lower pivot, in inches
- F is the application (piston) force, in pounds

we then arrive at this formula. This MUST be used on the forward (primary) shoe of whatever axle. Note: the angles are degrees.




Knowing the maximum piston force F, we can compare it to the piston diameter and line pressure, and see if it is appropriate. Adding another variable d_h for wheel cylinder diameter, and assuming a maximum line pressure of 1500 PSI, we can compute the contact pressure directly using a slightly different equation. (Note: a max pressure of 1000 PSI for manual brakes is more typical. Also, an additional consideration is that the maximum pressure that a driver can apply to the brake pedal and therefore brake system, is reliably proportional to their weight, excepting only those of a particularly athletic build).




Maximum Torque

We can find the maximum torque (in lbs.-ft.) on the shoe in question with the following formula:




Ideal Coefficient of Friction

The shoes are of the servo or self-energizing type which means that the rear shoe is subject to the same reaction forces both top and bottom as the front shoe. Because the coefficient of friction for brake lining does not change much based on contact pressure, we estimate that the overall torque generated by the rear shoe is the same as the front (the selection of lining was also different but at this date we don't know how it was different, so we will get an average value). (For reference, the worst case if no servo action were present would be a 30% overall reduction in torque.) Therefore the combined front wheel braking torque is simply 4T.

The weight of the car shifts to the front during braking and therefore the front wheels are designed to contribute more torque. We can estimate the ratio by examining the relative shoe and cylinder areas. The rear shoe areas are specified as the handbrake areas, while the total is also given. The hardware itself may also be measured. For example, a 55 Clipper has a 1-1/8 front, and 1-1/16 rear, wheel cylinder, for 11% less force in the rear. The brake shoe area calculation gives 20% less in the rear. For simplicity, we may just assume that the total braking torque is 185% of the front wheels torque.

Regarding the necessary coefficient of friction, we use the example of a 5562 which from the factory had a 60 to 0 distance of 151 ft @ 3925 lbs. (The brakes were engineered to not easily lock up the wheels, or if the wheels were to lock up, that the front and rear would lock at about the same time; therefore, the assumption that the brakes did not lock up during the test is reasonable to get an appropriate coefficient of friction, combined with our other data.)

Combining our existing data with well-known physical concepts, it is trivial to calculate that 151 ft. is 20.9 wheel rotations and the kinetic energy is 472 kilo-foot-pounds. Using the difference in kinetic energy, we find that a total braking torque of 3597 ft-lbs is required. Dividing by 185% then by 4, a torque of 486 ft-lbs on one front primary shoe would be expected for the required braking action (not including engine braking effects, which are negligible during an emergency stop situation.)

Combined Conceptual Example

Now for an example, the approximate variables for the front wheel on a 5562 are as follows. (Some are per the below drawing, which is very similar to 11-inch Clipper brakes).

- f, *to solve for*
- p_a, *to solve for*
- b, 2.5
- r, 5.5
- theta_a, 90
- theta_1, 20
- theta_2, 120
- a, 5.05
- c, 7.75
- F, calculated based on 1500 or 1000 PSI hydraulic pressure
- d_h, 1.125

We use this calculator that I set up, and input the variables. We find that for a brake line pressure of 1500 PSI, a frictional coefficient of 0.32 is required, and our maximum expected contact pressure is 175 lbs. For 1000 PSI, we need 0.41 and our contact pressure is lower, at 134 lbs.

Conclusion

Therefore, we predict that a product like Green Gripper Aramid Uncured (Non-Metallic Semi-Flexible) with a frictional coefficient of about 0.44, maximum speed 5000 fpm, and maximum pressure 100 PSI, should prove to be a reasonable choice in this application. Updating our coefficient of friction to 0.44, we see that our maximum PSI is exceeded and our line pressure should be limited to 700 PSI. What this means in practice is that the lining will have a reasonable life as long as the driver does not, on a regular basis, hit the brakes with more than 70% of "panic stop" force. A reasonable assumption on a manual brake car... less so on an Easamatic Caribbean.

For the heavier duty applications, a lining from McMaster with a pressure of 250 PSI would be reasonable to test. For example, a non-metallic lining with a 0.47 frictional coefficient and with 1500 PSI line pressure, only gets to 223 PSI contact pressure, while doubling our torque to 953 lbs.-ft. This lining would be acceptable in the application, though the reader may note that the higher-PSI lining is probably physically harder, which will increase both break-in time and rate of drum wear, so should not be used except as necessary.

Attach file:

---

  496926308_9437927496336263_1775935693530240582_n.jpg (380.05 KB)