Pump Talk
By Gary Handwerk

A Look At 10-Bolt Pto Pump Systems
When many of us hear talk about power take-off drives, we automatically think about small gear boxes bolted to the side of truck transmissions. When we hear about pto-driven pumps, we think about small pumps hung under the fire apparatus. Historically, that’s exactly what we bought in the fire service.

These ptos were readily available, low cost and were held on the side of the transmission by six bolts. Side-mounted eight-bolt units were only a little better.

About 15 years ago, Allison Transmission launched its World transmission with a 10-bolt pto opening. Lots of rumors and some promises were made about full power capabilities.

We started seeing apparatus with major 1,250- to 1,500-gpm pumps being side transmission pto driven. This was the first time we saw a pto drive with a large fire pump selling in any numbers.

Trucks with this configuration were built, but they may not have been well suited for the demands of the fire service. Some apparatus experienced pto failures within four years, and some had pto clutch failures during new apparatus acceptance testing.

It’s no wonder people in our industry, from apparatus manufacturers to dealers to firefighters, are confused about ptos and pto-driven pumps. And here’s why. We were looking at either pto pumps that were small and limited or so large they caused problems. There are questions about how big is the largest pump ptos will drive successfully and how long will the pto drive last.

Tapping Into Power
The word pto in the truck chassis/utility vehicle industry means any point where chassis engine power can be tapped to power something other than the chassis. By definition, a common truck chassis can have most, if not all, of the following ptos: front engine ptos used on front-mount pumps; rear engine ptos, like those used on the Pierce PUC; flywheel ptos, which are also called sandwich ptos, commonly used on fire apparatus in China, Japan and Korea; side transmission ptos, which is the kind we most commonly see here in the United States.

Apparatus can also have rear transmission ptos, which are commonly used on fire trucks in Europe.
And there’s the ever popular split driveline pto, which powers our old faithful midship fire pumps. That’s a transfer case with a rear top pto.

They All Work
As you can see, most of these ptos drive, or have been used to drive, fire pumps. The question remains as to why some are more commonly used than others. They all work, if applied correctly, and the system is designed for the service intended.

It all comes down to availability of the pto on popular chassis, market demand for specific pump performance and a drive’s ability to provide it, reliability, access to service and parts and of course cost effectiveness. As an example, front-mount pumps were 35 percent of the market, or more, at one time. Now less than one percent have front-mount pumps, and the obvious question is why did that happen. The answer lies in the limited chassis pto availably, and the fact that most builders charge more for a front-mount installation than a midship-split drive line setup.

Today, the most common alternative to the midship-split driveline setup is the 10-bolt pto system found on Allison automatic transmissions and Ford’s manual transmission found on its F550 cab and chassis.

So, let’s focus on the side transmission pto in general and the 10-bolt unit on the Allison transmission in particular.

Historically, manual transmissions were the norm in all fire apparatus, from a mini pumper all the way to an aerial. These transmissions had 6-bolt pto openings on the side, which had very little power or speed capabilities. All they could muster was sufficient power for a 250-gpm pump at the maximum. This was OK for a mini pumper, grass truck or for adding pump-and-roll to a full pumper.

When the ‘70s hit, we all went crazy for mini pumpers, and these chassis were equipped with manual transmissions. We installed 375- to 450-gpm pumps on them and tried to supply water at fires, only to burn up truck transmissions and ptos. The manual transmission was not the way to go. Some of these failures were gears breaking, oil burning up, even catching fire, and complete gear box and pto failure.

Pushing It Hard
Even today, the F550 manual transmission is very limited (500-gpm is pushing it hard) and has had some problems.

In the ‘80s, the Allison automatic transmission became the dominant setup. These transmissions had a 6-bolt opening, and we used a 270 series pto which had a 130-percent ratio and 250-foot-pound intermittent power rating. Add in the pressurized and cooled oiling system, and we had a very good 500-gpm complete system. This setup was common on tankers and midi pumpers. Yes, there were other pto setups, but they were not available on the more common chassis or cost more than their benefits warranted.

Starting in the early ‘90s Allison launched a new World automatic transmission, which quickly dominated our industry. In North America, more than 99 percent of apparatus over 20,000 pounds gross vehicle weight rating (GVWR) are so equipped.

Split Driveline Driven
Allison World transmissions drive nearly all the major pumps in North America today. By major pumps, I mean 750-gpm or larger as rated by NFPA 1901. At least 80 percent of those delivered in 2007 were split driveline driven pumps. This gives Allison’s automatic transmission 10-bolt pto drives for fire pumps a solid second place with an approximate 15 percent market share. Most apparatus using the pto systems are pumper/tankers or are interface or midi apparatus equipped with a 750-gpm or 1,000-gpm pump.

Accordingly, the 10-bolt Allison pto system has become a reasonably popular pump drive because it’s already there on the transmission everybody is already buying for apparatus. It also offers a larger pto than the industry is used to at a reasonable price.

Unfortunately, when the Allison World transmission was first launched, there were rumors and some flat-out incorrect statements made suggesting the pto system would be capable of full engine power. The fact is, however, a pto system is only as good as the gear in the transmission that drives the pto gear.

So, a small transmission gear equals a small pto, which equals a small fire pump. The gear in the Allison transmission is rated for 585 foot pounds of torque. That’s the highest possible pto rating.

Another limiting factor is the pto itself. It has gears, bearings, shafts and a shifting clutch. Ptos have continuous duty and intermittent duty ratings. They also have a life rating in hours, which is based on load usage over time and over speeding a pto, in an attempt to get more power out of it, will reduce component life. Finally, the system’s oil temperature must be controlled and the pto output driveline should not subject the pto to unacceptable loads. The chassis engine also should not subject the pto to unacceptable cyclical irregularity.

To better understand this, let’s look at pto ratings first. Ptos are rated as intermittent and continuous duty. Pto makers Allison, Chelsea and Muncie do not publish specific load life data and only list intermittent duty torque ratings. The Society of Automotive Engineers (SAE) does, however, set guidelines, and the three pto companies do participate in those meetings and use the guidelines.

According to SAE, intermittent duty means performance at that power level should not be exceeded for more than 5 minutes at one time and not for more than a total of 20 minutes per hour. Unless the fire pump will only pump off the onboard water tank, intermittent duty will not work out. 

To find the continuous duty rating, you need to de-rate the intermittent duty rating by 20 to 30 percent. At this continuous duty rating, SAE expects the pto to last 100 hours. A test done in the early ‘90s with a 10-bolt pto and an Allison 3000 transmission showed that 120 hours was possible.

Upon hearing that fact, you’re probably thinking to yourself: “Only 120 hours, that can’t be. That’s only 12 hours a year for 10 years.” That’s the bad news.

But there’s some better news, in general terms. If the pto is operated at half the continuous duty pto rating, the hour life span goes up approximately five times. And, if it’s operated at 25 percent, the life span increases by 25 times.

For example, a typical diesel engine at 300-hp or more, with an Allison 3000EVS transmission would handle a 1,000-gpm NFPA 1901-rated pump. It could be operated at capacity 5 hours annually for 10 years. At 500-gpm it could be operated for 20 hours annually for 10 years and at 250-gpm, 75 hours every year for 10 years.

This setup would be just fine for an apparatus used primarily for car accidents, Dumpster fires, grass fires and even an occasional initial structural attack.

However, it would not work well, or for very long, as an aerial water way feed pump, as a large diameter hose supply apparatus or for long duration deck gun operations. Departments using pto drives for large pumps would quickly run out of component life hours. Worse, while operating at high flow from a low-pressure hydrant, ptos can easily be overloaded causing immediate failure of the pto clutch or gears.

There are some promising alternatives, including a new automatic transmission from Caterpillar which has a rear pto that’s even larger than the 10-bolt pto on the Allison transmissions.

REPTO Is Another Option
The other option is the rear engine pto (REPTO) which is offered on some select engines and chassis models. It’s a live pto used on cement mixer trucks to drive the hydraulic pump. REPTOs have high power output capacity and long service life, but require an external clutch setup. And, last but not least, there’s always the front engine pto option, like those found on the International 7400 cab and chassis.

So, the upshot to all of this is there are some fundamental rules that must be followed when looking at pto-driven pumps. First, there are no magic pto-driven pumps that take considerably less power to create performance equal to that of a midship split driveline setup. Physics is physics.

Next, a small transmission gear equals a small pto, which, in turn equals a small pump, and all chassis drive sources have power input, power output and hours of life span.

Other Axioms
Some of my other axioms include the fact that a poor driveline design will affect pto and pump life, and torque in foot-pounds equals horsepower times 5,252 divided by RPM. Additionally, cyclical irregularity is very real as the engine creates vibrations at lower RPMs, which can cause unexpected component failures.

Lastly, components built by the thousands generally will cost less than equal components built in small quantities

If you’re still in doubt, just look at history. If nothing has changed, expect the same results. Pumps and drive systems built over many years and by the thousands should have a service and performance history to evaluate.

Editor’s Note: Gary Handwerk is global pump product manager for Hale Products. He has been involved with the fire service industry for 36 years working for various fire apparatus or pump manufacturers and has been a member of the National Fire Protection Association (NFPA) Fire Apparatus Standards Committee for 15 years.