The Stovebolt Site Forums The Parking Lot Big Bolts The Stovebolt Volunteer Fire Department Need a definition

Somebody help me explain the difference (other than the obvious) between a front-mounted pump and a midships pump.

Why would you want one or the other? And why do you not see as many "bumper pumpers" anymore?

Thanks,
John

Obviously we haven't archived this forum yet ...

Probably, the midship pump is hooked to the output shaft of transmission to get a higher speed and different torque? On my truck the differential is out of gear when pumping so you can put all the horsepower of the engine to the pump.
My thoughts...I dunno
Larry

IMO: NFPA sets the standards. With the cost of fire trucks and the hotter/bigger, faster burning fires we face today, they need to get as much water flow (GPM) as possible so they have gone away from the low gpm bumper mounted units.

Back in the day our trucks had to do everything but today there are special brush trucks and mini-pumpers that are low gpm units. All most all now use a midmounted pump to get 1500-2000 GPM. My old 1951 was a 500 gpm pump and it was top of the line equipment back in the 50's.

I hope that helps explain it...

Okay, that's a start -- thanks! Makes sense That front mounted pumps are limited by engine rpm and thus max out at low GPM.

Now what about "Pump and Roll" capability? Is that only important for brush trucks fighting grass fires? Is there any other reason you might want the P&R capability? Can you get P&R with midship mounting, or are you limited by the PTO?

Thanks for helping understand this,
John

I think you can get "anything" on a new rig including P&R as long as your willing to pay for it(big$$$). Locally in any of our six cities, I do not know of any rigs that have a pump and roll set up. We have brush trucks that we use in any such needed role. The engines are all static pump set up.

The airports and military air stations crash trucks have the pump and roll feature and are capable to stay right on board and fight the fire from a seat and attack with the monitor remote control. And are also equipped with 4-wheel drive to be able to roll over almost any terrain it encounters.

John,

Wow, you're making me actually feel "smart" for a change - I actually can answer some questions!!!

A front-mount pump is typically (not always) a smaller pump, driven off the front end of the drive shaft on a conventional (hood forward) truck chassis. The pump has its own, self-contained tranny, and, because it's not involved with the truck's transmission at all, offers the pump and roll possibility.

This is no small feature to a rural department fighting wild fires often. The literature touts "pump and roll, for fighting running grass fires!", and shows a fire fighter sitting on the front fender of the truck, with a booster line, spraying ahead of the truck.

The literature will also tout several other benefits of the front-mount pump, several of which I can vouch for.

1. Front-mounts offer excellent ability to draft water. Just nose the truck to the edge of a pond, lake, creek, pool, whatever . . . hook up the hard suctions and dump 'em in. The drive wheels stay on solid, dry ground. Simplified plumbing means far fewer opportunities for "air leaks", which prevent pump priming and drafting. With a mid-ship, you would typically have to "parallel park" against the water, which can isolate the engineer, provides less space between the truck and water, and is logistically less desireable.

2. Virtually always, a front-mount has fewer controls, and is far simpler to operate. Again, partly this is due to the elimination of the transmission interference issue, and also because these pumps are (were) rarely "two stage" pumps. A two stage pump has two impellers rather than one, and a more complicated case offering pumping in "volume" (both impellers pumping equal amount at equal, lower pressure), or "pressure" (one impeller pumps half-as-much water at one pressure, into the second impeller, which doubles the pressure, delivering half the volume.) For the engineer, this can add many complications to determine which is the better mode in which situation.
Not only that, but you can usually see all of the discharges on the front-mount, so there's no guessing about whether you're opening the captain's side discharge, the rear left, or rear right discharge, etc. etc.

3. Manufacturers claim better weight distribution, as well as leaving far more room for the water tank and/or equipment behind the cab.

NOW, a Mid-ship pump is typically mounted aft of the transmission, with a separate pump transmission which chooses between the pump and the rear differential. A mid-ship is engaged by pushing in the clutch, shifting the pump tranny from "Road" to "pump", shifting the truck into high gear (4th or 5th), and releasing the clutch. There's usually a locking mechanism to prevent the shifter from jumping out of gear.

Benefits:

1. Mid-ship mounts can be MUCH larger, since you can hang more weight between the wheels, than beyond the front axle. The drive-shaft arrangement provides the beefier power delivery required by a heavier pump pushing more water. This means greatly increased pumping capacity.

2. The larger pumps can offer the above-mentioned Series-Parallel (Pressure vs. Volume) capability, which allows a competent engineer to choose the best mode and lowest engine speed for a given pumping requirement.

3. The central location offers infinite plumbing options, including a deluge gun right above, and discharges anywhere you can point a finger.

4. Most would say the mid-ship pump is far better protected than the front-mount.

I've always said a front-mount is the best possible introduction to antique fire engines, because of their small, approachable size and absolute simple operation. Perfect for musters, too.

Mike Roache brings up a good point. Fire Engines have been built in every possible configuration. The airport crash trucks can flow tremendous amounts of water and foam while moving. But they cheat, by having a dedicated engine driving the pump only. And a mid-ship pump can be driven off of a PTO if desired, which would offer pump and roll, but PTOs are generally not sturdy enough to drive as large a pump as a traditional mid-ship arrangement.

And then there's American Fire Apparatus, which in the '60s and '70s mounted a full-size, mid-ship style pump, ahead of the engine, in a cab-forward chassis. I'm not sure how they squeezed the driver and officer into the cab . . .

I can't believe I actually wrote all this junk! Nobody could care this much about this stuff - I probably need to get out more . . .

Mike(s) -- Thanks for the info. Actually, call me weird but *I* find this discussion fascinating and educational. My pump is front mounted and I can see the benefits you mention.

This is all good stuff and I will include it in a revision to the Firebolt Tech Tip.

Thanks,
John

Mike - Thanks for all the info . I'm not quite the fire nut ( yet ) as you but one can not have too much info when it has to do with your interest . ED H

Don't encourage me too much, or I'll get out the projector and diagrams . . . .

-Engineer Mike

Bring it on. Tell us about the older style pumps -- the big things with big round chambers I see on '20's American Lafrances and what not. What kind of pump is that and why don't they use them anymore?

John, I think you mean like the Ahrens Fox pumpers like the one that was at Macungie last year..
I would be interested to know this worked.
Like this, (sorry I don't know how to shorten the link)

http://images.google.com/imgres?imgurl=http://www.firegold.com/images/FFfoxpump.jpg&imgrefurl=http://www.firegold.com/fox2.htm&usg=__jyxoSV_v2Cswb5FEOZs7TcOiqd4=&h=361&w=480&sz=34&hl=en&start=52&um=1&tbnid=tVAltUJIgYBSLM:&tbnh=97&tbnw=129&prev=/images%3Fq%3Dahrens%2Bfox%2Bfire%2Btruck%26start%3D36%26ndsp%3D18%26um%3D1%26hl%3Den%26sa%3DN

Yep, you jumped straight to the top of the heap with that one! Ahrens Fox was considered the Rolls Royce of Fire Engines. The company started in the days of horse-drawn steamers, mid-1800s-ish. Large boiler used coal-fire to heat water into steam, which, given the tremendous expansion abilities of steam, gave great pressure. This pressure was, in turn, put to work driving the fire (water) pump. Usually, but not always, a piston-style pump.

Ahrens Fox developed a piston water pump which, through intricately casted pump housings, used one-way valves to allow the piston to pump on both the down AND up stroke, gaining more work, and more consistent pressure, in the process.

Now, as for that famous dome:

Picture a piston pumping water in a cylinder. As long as the piston is moving, it's pushing water out of the pump. BUT, every time the piston finishes its stroke, it has to STOP, and change direction. This creates quite a pulsing in the fire stream - not desirable for the nozzle man hanging on to a 2 1/2" hose pushing 250 gallons per minute through a smooth-bore nozzle at 50 psi. Enter the dome.

Water will not compress. Air, however, WILL. The dome is placed in the discharge manifold of the pump. When the piston is pushing water, some water is pushed into the dome, compressing the air in the dome. NOW, while the piston STOPS and changes direction, that compressed air forces the water in the dome back out the discharge. When the piston starts pushing water again, (stroking the other way), the pressure again compresses the air in the dome. This happens over and over, very quickly, as the pump is working.

The net effect is that the water discharge is much smoother, providing a steadier stream, and making it a little easier to hold onto the hose. It still pulses some. It's pretty cool to feel actually. (Today's centrifugal pumps use the spinning impeller to throw the water out, in a consistent, circular motion, ergo, no pulsing.)

Ahrens Fox used the same basic pump when they moved to motorized fire apparatus in the 1900s, and placed the pump in front of the engine (Yeah, I guess technically, it's a front-mount pump). The Ahrens Foxes were the most expensive fire engines, and had the best reputation, always capable of outperforming their rating. But all that hardware up front - the pumps were 3 to 6 cylinders - made the trucks not only incredibly long up front and very heavy, but harder than most to turn. Ahrens Fox sold out to the CD Beck company (I think) in the '50s, which in turn was bought my Mack. The first Mack cab-forward fire truck - the "C" model, was basically an Ahrens Fox design. In fact, I think two were made with the Ahrens Fox badge. These were, of course, centrifugal pumps by this time.

By-the-way, American LaFrance made steamers as well, both piston and rotary pumps. Their rotary pumps also used pressure domes (smaller ones) to smooth the water delivery.

When I get my first Ahrens Fox, I'll let you guys all drive it!!!

-Michael

Excellent explanation Micheal.
Those Ahrens Fox fire trucks are beautiful, truley works of art. The one we saw last year at Macungie was museum quality and never made it off the trailer that I am aware of but it got as much attention as anything there. I think I have a shot of it somewhere, I'll see if I can dig it up.

Billy

Mike
Your post were great info on fire pumps. I felt like I was back in Operators school.
Great job
Dennis

Billy -- That's what I said -- Ahrens-Fox

Yeah, Thanks Mike!! Here we go with another revision to the Firebolt Article... Billy, if you find the photos of the truck at Macungie (which was the one I was thinking of, by the way), I'll use it with the article.

I'll give you a break from writing to finish up the SVFD logo

Regards,
John

Thank you all for the kind comments! you all are good for my ego - I think I'll keep you around!

Clearly, I could discuss fire pumps all day. Too bad the St. Louis Fire Department decided I'm too old to apply!

As for the logo, yeah, you'd think an officer of the SVFD would get faster results from the art department. :mad:

Well, I'll throw my rank around tomorrow and see what excuses come my way!

When I get around to it, I'll post some photos or ads of Stovebolt fire engines. But FIRST, the logo.

Stay tuned.

-First Engineer Mike

John, so far this is the only other picture I can find. I will keep looking.

http://i99.photobucket.com/albums/l316/Escape2002/Macungie%20PA%202008/IMG_3529.jpg

I liked that Mack!

as was said earlier front mount pumps are great to nose into the water supply and supply another engine in a relay or for filling tankers whether its 50' away from the water supply or the whole hose bed up to 1500'. this now can be acomplished with a mid mount pump with a front intake which is very common in this day and age. a front mount pump is/was desirable for drafting out of portable ponds whether it be one or by using a siphon for a two pond operation without tieing up the unloading areas of the ponds while it discharged up the lane suppling the attack engine. a mid mount with a front suction now replaces that set up with bigger pumps supplying the attack teams with more water when the water is avaiable. when front suctions first came about they were fixed usually flush mounted in the bumper. today they are mounted on top of the bumper with a 180 degree swivel. earlier pump ratings were less at the front then at the main side intakes because of the reduced size and plumbing of the pipe up and over the front axle. today that is not an issue because the trucks are bigger and higher which allows for a bigger pipe plumbed to the front. i still know of some nice size front mounts that are bolted onto the front of some GMC TOP KICK's.

pump and roll capabilities work best for field fires and huge wet down areas and are limited by the amount of water they carry. we had a 87 mack with a 1800 gal oval tank with a 750gpm american midship mounted tanker with pump and roll capabilities from a seperate pto. pump discharge was depended on the speed of the truck because the pump and truck was controlled by the same throttle.