Pump-and-Roll Capability For Brush Fire Trucks
For extinguishing grass and brush fires, a highly mobile vehicle which will fight fire “on the run” is desirable. In addition, the truck should carry shovels, brooms and back pack extinguishers.
For the department which only occasionally fights fires in brush and fields, a multipurpose structural pumper with pump and roll capability should be considered. It is recognized that the desirable characteristics of a brush truck may vary considerably from one locality to another. In addition, much compromising may have to be done if the individual department feels its need is for a structural pumper with brush truck capability instead of a brush truck with structural fire fighting capability.
In the final analysis, it is the primary duty of the truck that dictates what major and minor requirements are written into the specifications. This article will remain as neutral as possible.
Pump and roll
Ability to pump and roll should be considered as the prime requirement for a brush fire truck. The ability to maintain pump pressure and discharge volume is necessary not only on level terrain, but also while the truck is ascending steep grades. Other recommended features of a brush truck are all-wheel drive, low center of gravity for side slope operation, high flotation, adequate approach and departure angles, and sufficient inner axle clearance. These features relate primarily to off-highway operation. In addition, the truck should be able to attain relatively high speeds for getting to a fire, be able to maintain a constant pump discharge at varying vehicle speeds, provide adequate and safe riding positions for fire fighting personnel, and have brush guards to protect the radiator, lights and sheet metal.
Various methods have been used successfully to drive a pump for pumpand-roll operation. Perhaps it should be pointed out here that the power to drive most midship pumps is taken from the output of the truck transmission by inserting the pump transmission in the drive line between the transmission and the rear axle.
The pump gear ratio is usually based on the transmission being in direct drive. This means that the pump discharge will be affected by the shifting of the transmission into lower ratios for off-highway use, rendering it ineffective for pump and roll.
The front-mounted pump, which is driven from the forward end of the engine crankshaft, lends itself to brush truck work. Platforms are sometimes provided on each side of the pump for hand line operators. The disadvantage of this arrangement is that the pump and operators are exposed to possible injury. Also, the approach angle of the truck is lessened by the front overhang, limiting its maneuverability.
Flywheel power take-off
Another common pump drive for a brush truck is a flywheel power takeoff. This PTO unit is inserted between the engine and the transmission and runs at a constant speed with the engine. This type PTO is usually capable of handling full engine torque, so it is entirely adequate for full-size fire pumps. Like the front-mounted pump, a clutch is required for engaging and disengaging the pump. For a brush truck, it is desirable that this clutch be operable from both inside the cab and at the pump operator’s position while the engine is running at pumping speeds.
A skid-mounted engine-pump package, since it is entirely separate from the truck prime mover, lends itself to pump-and-roll operation and has been popular in aircraft crash trucks. There are various sizes of engine-pump packages. However, on a relatively small truck, such as a brush truck, weight and space are factors which must be considered, since it is desirable to carry a maximum amount of water.
Good pump-and-roll operation has been achieved through the use of a two-speed pump driven from a fulltorque PTO mounted on the transfer case. The transfer case is used on allwheel drive trucks to transmit power to the front wheels as well as to the rear. The two ratios of the pump permit normal pumping when the main transmission is in direct drive, and allow shifting the pump to a faster ratio when the main transmission is shifted to a lower (and slower) gear.
Probably the simplest method of achieving pump and roll is through the use of a small booster pump driven from a PTO attached to the side of the truck transmission. These pumps supply adequate volume and pressure to the relatively small nozzles used in fighting grass and brush fires. This arrangement will satisfy the need for a constant speed to the pump, regardless of the transmission gear being used, and may be an inexpensive method of converting existing equipment to pump and roll. It should be pointed out, however, that the converterdriven PTO outlets in most automatic transmissions in use today will drive the PTO at different speeds in the various ranges. Also, the PTO drive gear is large, resulting in fast PTO speeds— limiting the speed at which the engine may be operated with the pump engaged. This seriously hampers the offhighway performance of the vehicle.
Larger vehicles can be equipped with an Allison HT-70 automatic transmission, which offers the advantage of the flywheel PTO by using an optional top-mounted PTO.
Tilt cab advantages
A four-wheel drive chassis with a tilt cab lends itself very well for conversion to a brush truck. The tilt cab offers not only the advantage of good visibility, but also has better weight distribution because it carries more of the vehicle weight on the front axle than is possible with a conventional engine-ahead-of-the-cab chassis. With the weight more equally distributed between the front and rear axles, the all-wheel drive will perform better where traction is poor. In addition, by using either oversized or the newer wide base tires (single, rear and front), the vehicle mobility is further improved through higher flotation (less tire penetration of the ground).
An automatic transmission is a desirable feature since it frees the driver from the need to shift and gives excellent pump-and-roll operation on grades through the torque multiplication of the converter. It also gives the driver good vehicle speed control not only through the transmission gear ranges, but also by using the vehicle brakes, causing the converter to slip and allowing the engine speed to remain constant. This type of truck is versatile since, with foam equipment, it can serve as a good small aircraft crash truck.