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Much Debated Question for fun about a Forklift

  1. Sep 4, 2015 #1
    The question being is it harder for a forklift to lift the same weight farther out than in closer? An example say you put 3000 lbs on the tip of the forks, does that require more effort/strain/hydraulics to lift vs 3000 lbs positioned closer to the lifting cylinder?
     
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  3. Sep 4, 2015 #2

    Drakkith

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    I believe the work required to lift the load is the same, but more stress is put on the tines and it will take more force to tilt the tines upward if the weight is further out.
     
  4. Sep 5, 2015 #3

    Baluncore

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    To a Physicist NO, to an Engineer YES, but only very slightly.

    The moment acting between the carriage and the mast increases as the load is carried further forward. That increases the friction component of the mast rails and carriage rollers, so cylinder force required to lift will be very slightly increased with the weight forward.

    As the weight moves forward, load is transfered from the rear wheels to the front wheels. The front of the truck becomes lower as the tyres compress, so the truck must lift the load further to lift it to the same height. Does that lifting further constitute “harder”?

    There is no question that the forklift is more unstable when weight is supported on the tips of the tines. Once the rated SWL is exceeded, such that the rear wheels of the truck lift off the ground, the vehicle leans forwards, the lean will cause an increase in the torque and hence lean until the load hits the ground. Extended forks accentuate that effect, as does braking or accelerating in reverse with the load close to the tips or being carried high. If you pick up a load on the tips and then move forward, you may get a surprise when you try to stop and the load dumps itself.
     
  5. Sep 5, 2015 #4
    Ah yes great answers. My co-worker and I were talking about this and I said the lifting force does not need to be increased but I never thought of the mechanical aspect of it as far as the friction due to stress of the load torquing forward. So I guess were both kind of right. Also our forklift has solid tires they do not compress therefore do not have to lift any further. Its rated 3000 lbs @ 24" from where the forks begin which to my understanding is the safe distance before It will tip over. So my argument was it can lift 500 lbs just as easy @ 48" as @ 0". Then someone chimed in and said that putting the load out further would transfer weight from rear end (that being true) and the cylinder would have to lift that transferred weight. Then we all got confused!

    So I can go back and tell them that physically the force required does not increase based on how far out the load is but it does increase slightly on the account of friction due to the load torquing the moving parts.
     
  6. Sep 5, 2015 #5
    They do compress, even if you do not notice with your eyeball.

    The front wheels have to carry the weight of the load, and the transfer of the weight from the rear. The cylinder carries only the load weight.
    Since the front tires have more load on them, they compress a bit more, with the consequence that the cylinder has to move that little more.
     
  7. Sep 5, 2015 #6

    Drakkith

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    It's complicated. The vertical work done on the load is the same, no matter where's it's placed. However, placing the load at the end of the tines may increase the work the forklift has to do through increased friction, torque, etc. I'm no forklift expert, so I can't do anything but give some general ideas.
     
  8. Sep 5, 2015 #7

    Baluncore

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    The mast extends by telescoping sections that have rollers with roller bearings at the ends to carry the moment between mast sections. As the mast is extended, the separation between adjacent mast section end rollers becomes less. Hence the moment forces on those rollers progressively increases with extension, as do the resultant frictional forces.

    The carriage also has rollers. The fixed vertical separation of the carriage rollers is usually less than the length of the tines. So the forces on the carriage rollers may often be greater than the load being lifted. But, unlike the mast rollers, the carriage roller separation and forces are constant, so do not change as the load is lifted.

    It is all those rollers, along with the rough tracks on, and between, the mast sections that is the source of the frictional forces. The roller forces and friction will increase when there are greater lateral forces on the mast due to load being carried further from the mast axis, such as close to the tips of the tines.

    For more information dismantle a forklift mast or; Google 'forklift mast'.
     
  9. Sep 5, 2015 #8
    When you go back to your coworkers. You may look carefully at your fork truck. If not search out the lift chart for a crane. The work done is strictly a function of the weight raised a given distance. The Torque applied to the mast is a function of weight at a specified lever arm. 3000 Lbs at 24" is 6000 ft lbs of torque. If you were to extend the forks such that you could center your load at 6' from the mast the capacity of the machine would only be 1000 Lbs. but it could lift it easily. The only complication between a fork truck and a crane is that the boom on a crane swings in an arc. For a given length as the angle get closer to 90 (parallel to the ground) the lever arm gets longer as compared to the pivot. Crane tables are based on angle and mass while fork truck is based on load center and mass.
     
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