Illinois Railway Museum Blog

Let's talk about brakes and stopping of a streetcar.  Many grasp the concept that the motorman's brake valve sends air pressure to a cylinder.  That extends a piston and rod and somehow though bell cranks and more levers and rods, move the brake shoes to grip the wheel treads and stop the car.  As they say the devil is in the details.

A streetcar goes around some pretty sharp curves, and if a conventional system of levers was used, the act of that truck rotating and pivoting would serve to apply the brakes unintentionally.  In fact that is what happened on many early designs.  The designers came up with many ways to overcome this.  Basically some way to direct the force toward the center pivot of the truck, thereby isolating any unwanted forces, regardless of the orientation of the truck and the curvature of the track ti was negotiating.

Which brings us to our project car, TM 972.  They used a heavy circular plate, an arc or portion of a circle, hung above the rear frame member of the truck.  The brake rod from the cylinder came close to that and then was connected to a harp or something with rollers or sliders that could move left or right along that arc.  Thus, if correctly designed and installed, the braking for the truck would be relatively independent of its rotation.  The second part of the equation is how to release the brakes.  As air is released from the brake cylinder, there is nothing to make the piston return to its position of rest.  So again, perhaps more levers and springs to pull the brake shoes off of the wheel treads.

On our project car that system of parts was worn badly, almost to the point of failure.  Clearly the company in Milwaukee was not investing in anything but absolutely minimum needed maintenance and repairs.  Now that the new wheels are on, and the truck almost 100% rebuilt, the assembly work continues.  In the case of the above mentioned description we find that many parts need replacement. 

Spring Anchor

So here is the assembly for the return spring.  Four nicely machined NEW parts that fit perfectly!  I am constantly amazed at the depth of knowledge and skills of our volunteers.  You cannot go buy any such parts, or find an instruction manual on the internet.  Yet they have the mechanical common sense to understand the design, to determine if replacement is needed, to machine new parts and thus help insure long term preservation.  WOW!