The Pump Transmission – Putting the Power to the Pump



In the last installment of the “Anatomy of a Fire Pump,” Jay Rought wrote the article “The Pump Body – The Unsung Hero”.  In it Jay explains “the pump casing would be the “body” in relation to the impeller being the “heart”.  Just as the human body contains the heart and supports it by supplying passageways for the heart to do its job, the fire pump “body” contains and supports it’s “heart” – the impeller shaft assembly.” To compliment the pump body and impeller, the pump transmission is an integral part of the “Anatomy of a Fire Pump.”

The purpose of the pump transmission is to transfer the force of the apparatus’s engine/transmission to the fire pump while also increasing the speed to the pump impeller.  The majority of fire apparatus in the United States utilize a split-shaft driven transmission.  A smaller number of fire apparatus utilize a Power-Take-Off or PTO-driven transmission.  When using a split-shaft transmission, the apparatus has to be in a stationary position (vehicle in neutral with the parking brake applied). When using a PTO-driven transmission, the apparatus can be used for pump-and-roll or stationary pumping.

When Waterous developed the first pumper that utilized one combustion engine to drive both the vehicle and the pump, there was a requirement for a split-shaft transmission.  The split-shaft transmission has endured a number of changes since its inception.   Because power to drive the vehicle is transmitted through the split-shaft transmission, it must be able to handle the maximum level of torque that can be developed by the apparatus.  With the development of diesel engines and automatic transmissions, the maximum torque required by the driveline increased.   Increased rated volumes of pumps also required changes to the split-shaft transmission.  Early split-shaft transmissions used drive lines that were 1 ¾” in diameter and could transmit 4,100 ft/lbs of torque.  Diesel engines and automatic transmissions have required the drive shaft size to grow to 2.35” and have the ability to transmit in excess of 16,500 ft/lbs of torque.  Older pumps have been re-used on newer apparatus and without upgrading the drive shaft, some have twisted off the smaller inadequate drive line.

With today’s fire apparatus most common pump rating growing to 1,500 GPM and the maximum municipal rating growing to 2,250 GPM, the pump transmission has required the internal parts and the overall size to get larger.   Waterous has successfully reduced the overall size and weight of the split-shaft transmission by using lighter weight alloy aluminum in lieu of cast iron for the case that dissipates heat better.  Additions of diesel particulate filters in the exhaust system to meet new EPA requirements have taken up more space in the pump-house and required the reduction in envelope size of the pump transmission without changing the internal parts.

There is either a movable gear or a shift collar in the split-shaft transmission to move the transmission from road position to pump position and back again.  Because the teeth of the gear or the sprocket have to line up with the teeth of the sprocket or shaft without the aid of a synchronizer, some shift issues arose in the pump transmission.  Transition to the automatic transmission in apparatus also increased shift issues by adding torque to the moving parts required to shift from ROAD to PUMP.  Deeper pointing has been added to the teeth and sprockets of transmission to aid in pump shifting.  The teeth are also hardened to help alleviate damage that may happen from the operator failing to put the truck transmission into neutral before shifting the pump back into ROAD position.  In 2008 Allison Transmissions added software enhancements that improve engagements and disengagements of split-shaft pump transmissions.  The software affords a true neutral without the negative torque that would lead to shift issues.

Early split-shaft pump transmissions had manual shift linkages to perform the ROAD to PUMP and back action.  The pump shift has evolved from manual to electric to pneumatic pump shifts.  Pneumatic shift units allow for a higher shifting force and higher pressure lock of the transmission in either the PUMP or ROAD position.
When split-shaft pump transmissions were used with a manual apparatus transmission, the apparatus was usually shifted into 5th gear and the operator engaged a pin and hasp type system to lock the transmission in the pumping gear.  Automatic apparatus transmissions today utilize signals from the pump shifting process to lock the apparatus transmission in a 1:1 4th gear lockup pump mode.

Pump drive systems have evolved over the years to help prevent the accidental increase of the throttle while still in the ROAD position.  Requirements in NFPA 1901 now include three different safety interlocks before the pump panel throttle can be increased.  The three safety interlocks are confirmation of the apparatus park brake activated, confirmation of a completed pump shift and confirmation of 4th gear lockup pump mode of the apparatus transmission.

Gregg Geske, CAFS and Foam System Sales Manager, has been with Waterous for 21 years.  He has spent his time in the Service Department, as Applications Manager, and as the Central Area Sales Manager.