Bendix BP-1 User Manual

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Bendix BP-1 User Manual

®

SD-03-952

Bendix® BP-1 Brake Proportioning Valves

1.5 .340” MOUNTING HOLES (2)

3/8 P.T.

SUPPLY PORT

PIECE NO.

STAMPED HERE

I.D. TAG STAMPED

BP-1 FRONT OR 1/8 P.T. BP-1 REAR

CONTROL PORT

3/8 P.T.

DELIVERY PORT

FIGURE 1 BP-1 BRAKE PROPORTIONING VALVES

DESCRIPTION OF THE BP-1 VALVES

The BP-1 brake proportioning valves are incorporated into the air brake systems to improve the controllability and reduce the stopping distance of bobtail tractors during braking. BP-1, front and rear, valves reacting to the lack of pressure in the trailer supply line reduce the braking effort on the rear axles and increase the braking effort on the front axle of tractors operating in the bobtail mode. Treadle application force during bobtail operation resembles treadle application force during normal operation with a connected trailer.

A typical brake proportioning system is shown in Figure 6. The system consists of a front and a rear brake proportioning valve and a TR-3 valve.

Caution: The BP-1 front and BP-1 rear valves are similar in appearance but are NOT interchangeable. Note identification tag attached to one of the cover screws (Figure 1); it will be stamped ‘BP-1 Front’

or ‘BP-1 Rear’. The BP-1 rear valve can be used without installing a BP-1 front valve, providing the vehicle does not have a front axle ratio valve. If the vehicle has a front axle ratio valve, the ratio valve must be replaced with a BP-1 front valve.

OPERATION

A tractor that is equipped with the BP-1 rear brake proportioning valve and TR-3 valve only will respond in the following manner. With the trailer supply valve pushed in and a service brake application made, equal pressure (100% of service application) will be delivered to each axle of the tractor and trailer. When the trailer is disconnected from the tractor and the trailer supply valve is pulled out, the front axle of the tractor will receive 100% of the service brake application pressure and the rear axle or axles approximately 25%. As application pressure increases, the differences between the front axle and the rear axle(s) pressures diminish.

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A tractor that is equipped with both a front and rear axle brake proportioning valve with the trailer supply valve activated (tractor/trailer mode) will experience 50% of service brake application pressure on the front axle up to 40 psi. With service applications above 40 psi the differential between the supply and delivery diminishes until at 60 psi it is a one-to-one ratio. The rear axles receive 100% of the service application. With the trailer supply valve pulled out, (bobtail mode) the front axle of the tractor will receive 100% of service brake application and the rear axle(s) approximately 25% during normal brake applications. As application pressure increases, the differences between the front axle and rear axle(s) pressures diminishes.

BP-1 REAR VALVE (REFER TO FIGURE 4)

When the trailer supply valve is activated, air pressure is delivered to the trailer via the supply line. At the same time air is delivered to the TR-3 inversion valve which exhausts the air to the control port of the BP-1 valve and a brake application is made, air entering the supply port works upon the total area of the upper and inner pistons and delivery pressure equals supply pressure. See Figure 2, full delivery mode.

With the trailer supply valve released, air pressure in the supply line to the trailer is exhausted to atmosphere. At the same time the air pressure to the control port of the TR-3

 

FULL DELIVERY MODE

DELIVERY PRESSURE (PSI)

(NO CONTROL PRESSURE)

 

 

SUPPLY PRESSURE (PSI)

 

LIMITING MODE

(PSI)

(WITH CONTROL PRESSURE)

BLEND BACK

DELIVERY PRESSURE

POINT

 

SUPPLY PRESSURE (PSI)

FIGURE 2 PERFORMANCE CURVE FOR NOMINAL BP-1 BRAKE PROPORTIONING REAR VALVE

 

FULL DELIVERY MODE

DELIVERY PRESSURE (PSI)

(NO CONTROL PRESSURE)

 

 

SUPPLY PRESSURE (PSI)

 

LIMITING MODE

PRESSURE (PSI)

(WITH CONTROL PRESSURE)

 

DELIVERY

BLEND BACK

 

POINT

SUPPLY PRESSURE (PSI)

FIGURE 3 PERFORMANCE CURVE FOR NOMINAL BP-1 BRAKE PROPORTIONING FRONT VALVE

inversion valve is also exhausted. This sequence causes full reservoir pressure to be delivered to the control port of the BP-1 rear valve. This air is contained on the underside of the upper piston and when a brake application is made the air entering the supply port of the BP-1 rear valve can only affect the surface of the lower and inner pistons. Due to the reduced area of the lower and inner pistons, the valve delivers approximately 25% of the supply pressure to the relay valve. See Figure 2, limiting mode. In addition, spring #8, Figure 4 causes the lower piston to be initially held open until initial delivery pressure closes the lower piston and the valve will limit normally. The purpose of this initial delivery is to offset the relay valve crack pressure when the vehicle is operating bobtailed and enhance the performance of the proportioning system.

BP-1 FRONT VALVE (REFER TO FIGURE 5)

When the trailer supply valve is activated air is supplied to the trailer via the supply line at the same time air is delivered to the TR-3 inversion valve which exhausts the air to the control port of the BP-1 front brake proportioning valve. With air pressure removed from the control port of the BP-1 valve and a brake application is made, air entering the supply port works upon the total surface area of the upper and lower piston. The upper piston is restricted by the piston spring up to 40 psi; therefore, brake applications up to 40 psi can only displace the lower piston, which due to its surface area

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