It is a marketing film


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Posted by Gordon Maney [207.32.63.181] on Monday, November 04, 2013 at 08:24:16 :

In Reply to: Re: wheel cylinder question posted by Kaegi [98.247.164.246] on Sunday, November 03, 2013 at 14:15:19 :

A self energizing shoe may or may not be part of a servo brake assembly. The fixed anchor brake predates the servo brake. A fixed anchor brake shoe can and does self energize in a fixed anchor brake assembly. The question is, which shoe, and when?

A fixed anchor brake has one end of each brake shoe attached to the backing plate by means of an anchor pin. The shoe pivots on that anchor pin; the shoe motion is caused by the action of a hydraulic wheel cylinder applying a force on the other end of the shoe.

A fixed anchor brake may have one or two wheel cylinders. The drum brakes on WDX-WM300 Power Wagons and M37's use one wheel cylinder. [Many heavy truck brakes had fixed anchor brakes with two hydraulic wheel cylinders.]

A servo brake — a more recent design than the fixed anchor brake — does not have anchor pins. A servo brake will have one wheel cylinder. The lower end of the shoes will be spaced apart by a component called an articulating link. This link also provides adjustment capability, and is therefore frequently called an adjusting screw. A visible, physical feature of this adjustable link assembly is referred to often by the slang term "star wheel."

Each shoe in the servo brake assembly is attached to the backing plate by means of a retaining pin assembly including a part resembling a nail, along with a coil spring and two dissimilar washers. The retaining assembly applies a force on the web of the brake shoe, pushing it against the backing plate and keeping it from falling off onto the ground. If you removed the two retaining pins, both shoes and the articulating link assembly would fall off the backing plate.

Let us establish a couple of other terms first. Imagine we are looking at the driver's side of a truck equipped with drum brakes front and rear. We're sitting in lawn chairs looking at the front brake assembly with the drum removed.

Let's start with forward, leading, and primary:

The forward shoe is the one towards the front of the truck. The front of the truck, in the way I am meaning it, is the end of the truck that has the headlights.

The leading shoe is the shoe on the side of the brake that will arrive at the destination first, so to speak. If the truck is going forwards, the leading shoe is the forward shoe. If the truck is going backwards, the leading shoe is the shoe on the side of the brake that is toward the rear of the truck. In summary, the leading shoe leads the brake as it moves in the direction it is traveling. The other shoe is known as the trailing shoe.

The term "primary shoe" is only relevant to a servo brake assembly. The primary shoe is the forward shoe [and the secondary shoe is the rearward shoe].

The self energizing effect reduces required pedal effort. A brake shoe capable of the self energizing effect develops more braking power than one that is not, and less force must be applied on a self energized shoe than on one that is not.

Imagine a trailer that disconnects, allowing the tongue to drop to the ground. If the trailer is going forward, the tongue tends to dig in to the ground and stops rather quickly.

If the trailer is going backward and the tongue drops to the ground it will certainly drag. However, it will not stop the trailer nearly so rapidly as the first situation.

The tongue of the trailer in the going-forward situation could be likened to a brake shoe having the self energizing effect. The tongue of the trailer in the going-backward situation would not be likened to the self energizing effect.

Recall that the brake on our Dodge has a wheel cylinder at the top and anchor pins at the bottom. In that brake, one shoe will self energize — the question is which shoe? It is the leading shoe.

In a servo brake mechanism, the leading shoe self energizes, tends to dig in, tends to rotate around the perimeter of the brake, and in so doing applies the lower end of the trailing shoe. The force applied to the lower end of the trailing shoe causes it to self energize.

Servo action in a servo brake is the application of the trailing shoe by the lower end of the leading shoe.

In the servo brake, the forward shoe is also called the primary shoe. The primary shoe often will have a shorter lining, or it may have a different lining material than the secondary shoe. This is to enhance the operation of the brake when the vehicle is going forward, because that is the critical direction. We don't tend to have accidents while going backwards.

Fixed anchor brakes on many heavy trucks at one time used two wheel cylinders. That allowed both shoes to self energize when the truck was moving forward. When moving backwards, neither shoe self energized.

The video is fun and quite nostalgic. It cannot, however, be regarded as a piece of technical instruction media. Example — it happens to be very wrong in a critical way. They tried to suggest that the fixed anchor brake is superior to the servo brake. That is entirely wrong and absurd. It was a desperate attempt at marketing. Consider that every manufacturer made the move at one time or another from the fixed anchor brake to the servo brake. The reason being that the servo brake is far superior because both shoes self energize, not just one.

Either the upcoming issue of the magazine or the one following will begin a big article on brakes, by coincidence.






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