Let’s Stop RUNAWAYS
SAFETY
Engine 2 rolls up to the scene of the blaze. As the vehicle slows, the crew dismounts and begins to lay hose. The engineer sets the parking brake, shifts to neutral, engages the pump, and shifts back to drive. He dismounts and moves to the pump panel and increases engine speed to pump rpm (revolutions per minute).
Everything appears routine as the crew advances on the fire. Then, Engine 2 begins to move down the street—a 17-ton apparatus, out of control and on a rampage. These firefighters have a runaway on their hands.
Though not common, runaway vehicles are a problem. They cause damage to property and fire apparatus, as well as injuries to firefighters and bystanders. Most fire departments do not like to discuss runaway incidents.
According to estimates, there are anywhere from 8-25 runaway incidents every year, and damages exceed $500,000. This is in addition to the cost of personal injury, liability claims, and loss of equipment use.
The cause of runaway vehicles is technological advances: larger capacity pumps, automatic transmissions, and the split-shaft drive systems that often accompany them.
In most split-shaft applications, power is passed through the transmission during both road and pump mode. In road function, power is passed from the engine, through the transmission, and along to the drive axle.
A shift to pump operation is usually accomplished using a sliding gear and collar. The gear is moved forward, splitting the shaft. Power is passed through the transmission to the pump gear box. The split shaft prevents power from reaching the drive axle. A runaway results when this sliding collar and gear assembly fail to move forward and lock in the new position.
Lock-up failures are often attributed to operator error. However, vehicle operators are often scapegoats for mechanical or system defects. The split-shaft system has inherent torque and delay problems.
Usually, there is a substantial buildup of torque on the vehicle drive train when the parking brake is set. This buildup is especially evident if the brake is applied before the vehicle comes to a complete stop. The shift from drive to pump mode cannot take place when this torque is present.
A threeor four-second delay when shifting from neutral to pump mode will normally allow this buildup to dissipate. However, even with a delay, some torque may still remain.
Even without torque buildup, the shift to pump mode is not always certain. Most automatic transmissions have a shift delay. If the operator shifts rapidly to neutral, engages the pump, and shifts back to drive, the sliding collar may not lock in the pump mode position. The “OK-to-pump” light will illuminate. However, as engine rpm is increased to bring up pressure, the engine will eventually overcome the parking brake.
SOLVING THE PROBLEM
How can we eliminate this problem? One way is to increase operator awareness and training. But, the problem is not with the operator, it is mechanical. A better solution would be to eliminate the mechanical system that allows the runaway situation to occur.
We must replace the split-shaft drive and pump method with a system that is simple to operate and will be less affected by operator error or oversight. Two available options are:
- An engine driven power take off (PTO),
- A sandwich or flywheel PTO mounted between the engine and transmission.
Engine driven PTO
Engine driven PTOs bolt to a special housing on the transmission and are driven by a ring-gear at the flywheel. Power for the pump does not pass through the transmission. When the vehicle is in pump mode, power is passed from the engine to the PTO, which is engaged using a control that is usually located in the cab or at the pump panel. During drive mode, the PTO is disengaged and the transmission is placed in gear.
The engine driven PTO has two major disadvantages:
- It presents mounting problems.
- It does not provide enough power to drive large capacity pumps for long periods of time.
The engine drive PTO is mounted where the engine and transmission bolt together. This installation usually interferes with the normal mounting of both the alternator and power steering pump.
Most manufacturers are forced to provide special provisions for mounting these components when an engine driven PTO is installed. This increases the price of the vehicle. In addition, special installations may cause service or maintenance problems.
Two PTO units are usually used: SAE (Society of American Engineers) six bolt and SAE eight bolt. Neither one is rated to handle large capacity pumps for extended periods of time.
The six-bolt PTO is rated for approximately 250 feet/pound, which is sufficient to drive a 500gpm pump for most pumper applications. The larger eight-bolt PTO is generally rated at 500 feet/ pound and is sufficient to drive a 1,250-gpm pump intermittently. At best, it is borderline for continuous pumping at 1,250 gpm.
Pump sizes and capacities continue to increase to meet firefighting demands. Today’s large pumpers use 2,000-gpm pumps, and there will probably be even larger capacity pumps in the future. As it is now designed, the engine driven PTO cannot handle these large capacity pumps.
Sandwich PTO
A PTO that is mounted between the engine and transmission offers two distinct advantages over the traditional engine driven PTO:
- It can drive large capacity pumps.
- It presents fewer mounting problems.
The PTO is sandwiched between the engine and transmission. Power is transmitted from the engine to the PTO. At this point, power is available to drive the pump and/or the transmission.
A driver or operator controlled clutch within the PTO is used to activate pump drive. Power is transmitted through the PTO to the pump. During pump mode, the transmission can be in “park,” “neutral,” or even in one of the drive gears.
Under this system, the transmission has been completely removed from the pump drive line, eliminating any chance of a runaway vehicle.
Drive or road mode is activated by placing the transmission in one of the drive gears. The pump can even be engaged during road mode, offering pump-and-roll operation.
Sandwich or flywheel type PTO units can be rated up to an excess of 1,000 feet/pound, more than enough to drive 2,000-gpm (or even higher) pumps in continuous operations. This type of PTO has the power and drive capacity of a separate pump engine, without the additional cost or weight.
The input housing on the PTO is usually 8-10 inches. This leaves sufficient clearance between the rear of the engine and the PTO take-off housing for normal mounting of the power steering pump, alternator, compressor, and other auxiliary equipment.
The engine/PTO/transmission package is generally 20-30 inches longer than with a split-shaft installation. However, the overall wheelbase of the vehicle can actually be reduced because the pump can be mounted above the transmission when a sandwich or fly-wheel PTO is used.
This PTO is not a new concept. Crash/fire/rescue (CFR) vehicles have been using them for nearly ten years. In CFR applications, a modulating clutch is added to the unit and it is known as a power divider. The modulating clutch allows control of both pump output and vehicle speed for better control during pump-and-roll operations. There are currently more than 1,000 power dividers in active use.
The additional cost for a power divider or flywheel PTO is between $4,000 and $6,000. Cost, however, is not the major obstacle to more departments acquiring this PTO. Rather, it is the fire industry’s inherent reluctance to change. But, most technological advances occur when the end-user begins to demand something better.
SUMMARY
Firefighting personnel already face enough hazards on the job. They deserve the safest and most effective equipment available.
A runaway fire truck adds unnecessary risk to an already highrisk profession. The fire industry can help to stop runaways by changing the present split-shaft pumping system.
