Wedge Brakes on Trucks Have Many Advantages
In our column last month we briefly discussed brakes, hydraulic and air-actuated, as related to specifications for low maintenance, braking balance and operating safety. We will not spend time discussing the standard emergency brake, which is actually no more than a parking brake in most trucks.
The Interstate Commerce Commission, which regulates interstate trucking, requires that air brakes be supplemented by a mechanical device that will apply the brakes when air pressure drops below a safe level. This is a spring-operated device held in the release position by air pressure. While the low-pressure warning signals usually operate at 65 psi or less, some leeway is provided in the spring chamber design. When the air pressure drops to about 35 psi, the springs are activated. Thus, there is about a 30-psi drop before the spring chamber acts to apply the brakes.
Control maintained: Some fire departments, and the number is increasing, specify this safety feature on their air brake system. It means that the loss of air no longer means the loss of brakes or control. The cylinders are mounted on the rear axle as shown in Figure 1. This is a tandem axle assembly and the cylinders are shown on the front axle of the tandem. On a single axle and the rear axle of a tandem, the brake cylinders are mounted on the rear of the axle. These are not used on front axles.
In a fire department, these spring brake cylinders prevent the driver from moving the fire truck into the street unless he has at least 35 psi of air pressure. We have seen these spring brake chambers operate when an air line failed and they stopped the vehicle in the minimum distance for an emergency brake application.
Once the brakes are applied, they can be released by turning a manually operated handle, which is a rather slow process, to again compress the spring. The vehicle cannot be moved until this is done. Some fire apparatus manufacturers furnish with air brake systems an extra air cylinder equipped with check valves and a manually operated air valve to provide spring brake chamber release pressure. This eliminates the time loss of manual operation.
Wedge brakes: About six years ago a new type of brake actuation was introduced and has had a steady increase in acceptance in the heavy-duty automotive field. This is the wedge brake. It is offered as an option by at least three axle and brake manufacturers. While it is an option with most fire apparatus manufacturers, for some reason they are reluctant to supply it except on specification. In discussions, we can learn only that it is a personal reluctance and is not based on any actual experience.
Young Fire Equipment Co. photo
Actually, for the fire service, it has many, many features that warrant serious consideration for greater brake safety and less maintenance.
The conventional location of the hydraulic brake actuation cylinder is inside the brake drum, Figure 2. The conventional air brake actuating cam is shown in Figure 3. The wedge type of brake actuation is shown in Figure 4. Generally, two wedge assemblies are used, as shown. The exception would be on light vehicles when only one wedge assembly might be used. The principle of operation is the wedging action provided by a plunger moving from air (or hydraulic) pressure to move outward two sliding elements which apply pressure to the brake shoes.The outer end of the plunger is provided with two rollers which operate in a guide to eliminate any binding action.
The unit is packed with grease to require maintenance only once a year.
Advantages: The experience of large truck fleet owners is our best record of the kind of service we can expect for fire duty. The wedge brake record indicates the following advantages:
- Greater safety
- Less maintenance
- Longer lining life
- Quicker response with lower hy-
- draulic or air pressure
- Fully automatic brake shoe adjustment
- Reduced parts inventory
- Decreased weight of brake parts
- Spring brake chambers designed as combination unit with air brake chambers
Anything that improves safety and reliability of the braking system is worth consideration. The wedge brake requires less pressure to operate and gives a smooth application with driver control. No moving parts are exposed to corrosive conditions such as the weather and road dirt.'”With all moving parts protected and packed in grease, maintenance is at a minimum.
Longer lining life has been achieved by (a) increasing the air flow for greater heat dissipation from the drum and (b) the formulation of the brake lining. The brake drum diameter is 15 inches to give greater clearance with the wheel for increased air flow. An increase in the coefficient of friction of the lining also partly compensates for the reduced diameter of the brake drum.
Less pressure needed: One of the big gains is the reduced pressure requirements for either air or hydraulic systems. This reduced pressure, 50 percent less for air, automatically means reduced maintenance of the entire braking system, including the air compressor.
The long plunger travel possible in the wedge brake design provides for full lining wear without making an adjustment. The application of pressure is always uniform for the total lining area.
For fire departments having vehicles with both hydraulic and air-operated brakes, the wedge brake is valuable as all the brake parts are interchangeable for a given width of brake-shoe. This means reduced inventory and quicker service from the manufacturer.
Wedge brake lighter: While reduced weight may not be an important factor to most fire departments, it does contribute to reduced tire wear. It will be an important factor when manufacturers finally realize the importance of reducing vehicle weight to reduce costs.
The reliability of hydraulic brakes can be greatly improved with the wedge brake as the hydraulic unit (cylinder and piston) is entirely outside the brake drum. Thus, it is free from the heat normally present on long and severe applications. Also, the hydraulic brake cylinder is free of the limitation on diameter. Thus the limitation of 30,000 pounds on gross vehicle weight will not apply when the wedge brake is used.
Courtesy of Rockwell-Standard Corporation
The spring brake chamber is easily made a part of the air brake chamber design for the wedge brake. Figure 5 shows the complete and compact assembly.
As more wedge-type brakes are used in the fire service, we will be able to give you a maintenance and operating report that will be more representative of the fire service experience.