1. #1
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    Default PTO generator question

    Is it possible to have a "hot shift" PTO generator? Meaning, can the generator be turned on and used while the apparatus is still in motion? This would be used for lighting as we roll up to a scene. We've become quite spoiled with our hydraulic generator providing light as we pull up. There is a large difference in price between a PTO and hydraulic genset, not to mention maintenance, etc.. We are in the very early stages of specs, and I thought I'd ask the question.

    If it's possible, great. If it isn't, I'm wondering why. With the ability to "pump and roll" with PTO pumps, why couldn't the same be applied to PTO generators?

    Go easy on me, this is my first post . I've been reading this forum for about 3 years, and have gained knowledge and benefited from many of the threads.

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    The generator has to run a specific, constant speed to generate electricity at the proper voltage and hertz.

    PTO speed varies with the revolutions of the engine. With a PTO driven generator, the electronic controls of the generator govern the speed of the engine so the required generator speed can be maintained. You cannot drive the vehicle when the PTO generator is engaged, as the the PTO speed would be varying up and down as you depress and release the accelerator pedal, so the generator speed would be going up and down and would not provide the proper current.

    In a hydraulic generator, the PTO drives a hydraulic pump, and the generator is driven by a variable hydraulic motor. The amount of hydraulic fluid driving the motor is varied with a diverter valve depending upon the speed of the PTO so the hydraulic motor turns at a constant speed to drive the generator even when the PTO speed is varying.

    The reason the hydraulic generator is more expensive, is because of the need for the hydraulic pump and hydraulic motor that is not required in a PTO driven generator.

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    There are two basic types of hydraulic units. (none of what I write here applies to direct drive units)

    Those that have their PTO engaged all the time, and those that have shiftable PTOs.

    With a shiftable unit, you usually only have to idle down to 1,000 RPM or less. This assumes you're using a hydraulic hot shift. This type of unit uses high pressure fluid from the transmission to shift very quickly. If you use a manual, electric, or pneumatic shift, this isn't an option, and you'll likely have to stop, put it in drive, and then engage.

    My volunteer dept had a permanently engaged unit. You could engage the generator while riding at 60 mph with the engine at 2,000 rpm. Turning the system on only caused a valve to divert hydraulic fluid to the hydraulic motor. The downside to this type is that it generates a little extra noise anytime the engine is running, and if you rupture a pressure hose you'll likely run out of fluid and cause permanent damage before you can get home.

    You can have the best of both worlds. Have separate switches for the hot-shift PTO and the generator engagement. You can then leave the hydraulic system running, without having the lights on while riding down the road.
    The American people will never knowingly adopt Socialism. But under the name of 'liberalism' they will adopt every fragment of the Socialist program, until one day America will be a Socialist nation, without knowing how it happened. --Norman Mattoon Thomas, 6 time presidential candidate for the Socialist Party of America

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    Most systems out there through research, state to be engaged below 1000 rpm's.

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    Quote Originally Posted by txgp17 View Post
    You can have the best of both worlds. Have separate switches for the hot-shift PTO and the generator engagement. You can then leave the hydraulic system running, without having the lights on while riding down the road.
    This is how our new Toyne Rescue Pumper operates. The operation is through the V-Mux screen and one needs only to ensure the RPMs are 1000 or less, hit one button to engage (Generator Start Link)and a second to power the generator (Generator ON/OFF). From there you can select any of the body mounted lights while traveling at any speed, or await arrival as would be normal. This allows for instant lighting. The key is ensuring the initial engagement is done before the operator disembarks and starts the pump, as once pumping reducing the RPM's to 1000 may be an issue.

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    txgp17,

    We currently have two hydraulic gensets in our fleet. One, a 25KW AMPS, is always "in gear", and you only to use one switch to turn the generator on. The downside to this is that it's always engaged and it also takes some HP away from the motor. The 2nd one, a 10KW Harrison, uses two switches and is like RFDACM02's Toyne.

    I guess my original question might have been a little vague, but I was directing in towards the non hydraulic direct drive PTO generator (Onan, etc.).

    So, I guess I should change the question to a direct drive PTO generator and the ability to "hot shift" it while in motion.

    By the way, RFDACM02, love the Toyne rescue pumper. I know it was asked in the other thread, but is there any way you could post some specs of it? It's very similar to what we would like to spec in the next few years.

    Thanks.

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    PTO generators can only be used in a stationary mode ONLY. They also don't work that well if you have more than one component that is PTO driven such as an auxiliary PTO fire pump or PTO drive hydraulic rescue tool system. These systems are much more economical but you have to be stationary as the generator must run at a constant RPM.

    A hydraulic generator which is still powered by the PTO can be used while the truck is motion as long as it's engaged below a preset RPM (usually 900-1100 RPM). These systems work well on pumpers as the pressure governor usually has control of the idle on the truck so the generator really doesn't care what RPM the truck is at. These systems however are much more expensive, but you can do a lot more with them.

    The only time it makes sense to use a PTO direct drive generator is on a Rescue, and even sometimes there it is questionable.

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    Quote Originally Posted by toddmcbr View Post
    PTO generators can only be used in a stationary mode ONLY.
    That's incorrect. Ours worked as well at 62 mph in 5th gear at 2,100 rpm as it did at 0 mph in Neutral at idle speed.

    Driving, idling, or pumping, it worked at rated capacity and could be engaged, disengaged at any time.
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    Quote Originally Posted by txgp17 View Post
    That's incorrect. Ours worked as well at 62 mph in 5th gear at 2,100 rpm as it did at 0 mph in Neutral at idle speed.

    Driving, idling, or pumping, it worked at rated capacity and could be engaged, disengaged at any time.
    No, He's right. A PTO generator runs from the PTO and has an RPM governor to control the speed so that it maintains a set voltage. Hydraulic powered PTO generators are different

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    Quote Originally Posted by MG3610 View Post
    Hydraulic powered PTO generators are different
    No. He, and you, are wrong.
    A Direct Drive PTO generator, is a PTO generator.
    A Hydraulic Drive PTO generator, is a PTO generator.
    Last edited by txgp17; 03-19-2009 at 07:00 PM.
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    My volunteer dept had a permanently engaged unit. You could engage the generator while riding at 60 mph with the engine at 2,000 rpm. Turning the system on only caused a valve to divert hydraulic fluid to the hydraulic motor. The downside to this type is that it generates a little extra noise anytime the engine is running, and if you rupture a pressure hose you'll likely run out of fluid and cause permanent damage before you can get home.
    What you are referring to as a "permanently engaged unit" is actually a hot shift pto meaning the gear of the pto and the gear in the trans are always in mesh. When you "engage" the pto, you are engaging the clutch within the pto itself. That is why you must lower the rpm, for the same reason you don't engage the clutch in your car at 2000 rpm, it won't last.

    The reason you place shiftable pto's in drive before engaging the pto is to stop the drive gear within the trans, otherwise you'll grind a pound of steel.

    And as already mentioned, pto generators must run at a preset rpm in order to maintain the correct voltage and HZ.
    Hydraulic gensets have variable flow pumps that control the genset rpm, that is why pumpers have hydraulic gensets and not pto gensets.

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    Quote Originally Posted by VanIsleEVT View Post
    What you are referring to as a "permanently engaged unit" is actually a hot shift pto meaning the gear of the pto and the gear in the trans are always in mesh.
    No.

    Our unit was not a hot shift. It did not shift. It was engaged all the time, thus it was a constant mesh type, and it was not shiftable. If the flywheel was spinning, so was the hydraulic pump. You could hear & feel the fluid flowing in the hydraulic lines when the generator was engaged and disengaged. We verified this by touching and listening, with a mechanics stethoscope, to the lines with the engine idling and the generator off.
    Quote Originally Posted by VanIsleEVT View Post
    When you "engage" the pto, you are engaging the clutch within the pto itself.
    Not on our truck. The generator switch only diverted fluid to the hydraulic motor, instead of looping it back to the reservoir.
    Quote Originally Posted by VanIsleEVT View Post
    The reason you place shiftable pto's in drive before engaging the pto is to stop the drive gear within the trans, otherwise you'll grind a pound of steel.
    And that only applies to the older automatic transmissions that have turbine driven PTO's. The Allison World series uses flywheel driven PTO's. Putting them in drive while stopped does nothing to stop the PTO from turning, thus all shiftable PTO's on Allison World transmission must be hot-shift type. Otherwise you can only engage them when the engine is stopped.
    Quote Originally Posted by VanIsleEVT View Post
    And as already mentioned, pto generators must run at a preset rpm in order to maintain the correct voltage and HZ.
    That isn't limited to PTO generators. All generators must run at a preset rpm to maintain proper voltage & Hertz, unless they us some form of an inverter to make the necessary corrections.
    Quote Originally Posted by VanIsleEVT View Post
    Hydraulic gensets have variable flow pumps that control the genset rpm, that is why pumpers have hydraulic gensets and not pto gensets.
    It's still a PTO generator. The generator receives torque from the hydraulic motor, which receives fluid under pressure from the hydraulic pump, which receives torque from the PTO, which receives torque from the Apparatus' prime mover (main engine). By definition, that makes it a PTO generator.
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    You're clearly here to argue which I'm not going to do but having said that, the industry uses the terms "pto generator" and "hydraulic generator" when referring to the two different types most commonly used. Pto generators being the type used mostly on Rescues and hydraulic generators being used on Aerials and Engines.

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    All I'm going to say is, is if you try and order a "hot shift" PTO for a "continuous driven" PTO, your screwed.

    There is a difference between them.

    FM1

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    Try googling "ONAN GENERATOR" go to commercial generators, they give a very in depth description of each type there, as well as blueprint drawings that show how each is driven, the "pto" generator runs directly off of the trans. pto, the "Hydraulic" generator has a hydraulic pump which is run off of the PTO, this allows the engine speeds to change without affecting the generator because it is controled by the hyd. pump. not the engine rpm.

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    I think most of us understand the difference between a PTO and Hydraulic generator, but using the same thinking as a previous post, wouldn't they all be diesel or gasoline generators then? They are all technically driven by the vehicle engine which in turn is powered by diesel or gasoline.

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    Quote Originally Posted by VanIsleEVT View Post
    You're clearly here to argue which I'm not going to do but having said that, the industry uses the terms "pto generator" and "hydraulic generator" when referring to the two different types most commonly used. Pto generators being the type used mostly on Rescues and hydraulic generators being used on Aerials and Engines.
    I'm not here to argue, I'm here to substantiate my position. True, the industry commonly uses that nomenclature, but populism doesn't make something correct. A hydraulic generator, whose pump is driven by a power take off, is still a power take off generator.
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    Quote Originally Posted by txgp17 View Post
    True, the industry commonly uses that nomenclature, but populism doesn't make something correct.
    I think we see where you're coming from, but when the industry uses a widely understood term, how does that make us wrong? I spec rigs with "NST" couplings, even though the true name is "American National Fire Hose Connection Screw Thread." I think people still understand what I mean....
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    Default PTO Generator

    PTO driven generators were originally designed to be driven by the power take off on a tractor that operates at either 550 or 1100 rpm. They have to operate at a set speed with no variability to maintain the 110/220 or 440 volt output at 60 hertz.

    They have been adapted for use on mostly rescue type vehicles and have to set stationary with a preset fast idle speed that matches the transmission mounted PTO gear ratio to the 1,100 rpm. generator speed. Say .92 ratio on PTO gives 1,100 rpm on generator and about 1,200 rpm on vehicle engine. Thats how our unit is set up. Vehicle must be stationary before engaging the generator. If it is engaged while moving the variable speed can blow out the light bulbs in the 110 volt system.
    A PTO driven hydraulic pump driving a hydraulic motor driven generator can compenstae for the variable speeds.

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    My $0.02:

    Generators are referred by the type of drive to the genset. Hydraulic generators, though the pump may be driven via PTO or V-belt, have a hydraulic motor driving the genset. PTO generators have a power take-off shaft from the transmission directly driving the genset. Other generators can be driven via an auxiliary engine, V-belt or some other method.

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    This issue of Fire Apparatus Magazine has an ad from Raven Technology. They are introducing a PTO generator, 15KW that will operate at any engine speed. Interesting, we'll see if it works!

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    Thanks to all who posted. I have always refered to the two types as PTO and hydraulic, with the PTO being the direct drive and hydraulic having the ability to "hot shift." I probably should have been more clear in my original post and referred to the PTO as direct drive. Obviously now I see that it is not possible to have a "hot shift" direct drive PTO generator (Raven Tech notwithstanding). Thanks again.

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