Why do tower cranes have gear shifts?

Question

Internal Combustion Engine powered cars need multi-speed gear shifts since their power is highly dependent on speed and engines have narrow usable power/speed bands.

Some electric vehicles are also coming out with gear shifts. We have Antonov already working on a 3-speed gearbox for electric motor drive, and the new Exagon Furtive eGT is said to be coming with a 4-speed gearbox. Why multiple speeds on an electric vehicle? The reasoning is the same as for an engine. It is to allow lots of torque in first gear to start but also to eliminate the need for the motor to run insanely high RPM's when you're going 80 mph on the freeway.

Metal band saws also come with some form of mechanical speed control (CVT or* gear shift) since you will want to cut softer stuff using high gear, but you will need low gear to cut iron, steel, etc. There's Clausing Industrial who have put two xor THREE speed controls: a belt-drive CVT, 4-speed gearbox, and possibly an infinitely-variable** inverter motor, in their V4014H vertical band saw!

But what's odd is that many tower cranes also have gear shifts (on the hoist winch) even though they're electric!

For example, the hoist winch on the Liebherr 357 HC-L 12/24 luffing boom tower crane appears to have a 3-speed gearbox immediately after the motor. Using a 215-horsepower (160 kW) motor and 1-fall operation, the maximum no-load lifting speeds are (capacities based on halving the capacities of 2-fall operation):

• 330 m/min (3rd gear) maintainable up to 1,500(?) kgf
• 185 m/min (2nd gear)
• 113 m/min (1st gear)

And the maximum full-load line speed at 12,000 kgf (even on third gear, I presume) is 68 m/min.

The smaller 147-horsepower (110 kW) hoist winch also, apparently, has 3 speeds (these are again on 1-fall operation):

• 261 m/min (3rd gear) maintainable up to 1,100(?) kgf
• 157 m/min (2nd gear)
• 91 m/min (1st gear)

And the maximum full-load lifting speed (again, on any gear) is 45 m/min.

NOTES:

• "No-Load Speed" is at zero hook load (when the crane is lifting only the hook-block)
• kgf = Kilogram-force
• *Logical OR
• ** "Infinitely-variable" = "Continuously variable down to zero speed"

My questions are:

1. Despite the fast lifting speeds often encountered and the high load capacities, shouldn't the working range of a tower crane be smaller than that of a car?

2. If electric motors do so well on cars without a gear shift, why would you need such a thing on a winch? In this case, a crane hoist)

Answer 1

You need very slow speeds to make precise movements of the load. At high speed, the cables will stretch as the load accelerates and decelerates and then oscillate longitudinally, "bouncing" the load up and down (and bouncing the whole crane boom as well). That is not a good idea if you want to raise or lower the load accurately through a small distance (say 10mm).

Also, moving a large load a small precise distance vertically is different problem from moving a car a small precise distance horizontally, because the crane motor has to provide the force to counter-balance the full weight of the load all the time that the brakes are off, but the car engine does not. A lower gear ratio means the torque on the motor is less, and therefore the motor generates less heat while it is effectively stalled holding the load stationary.

You can easily move a car 10mm horizontally by pushing it with one hand, but you can't do the same thing to a 12-tonne weight hanging from a cable!

Just to avoid any misunderstanding, you don't "change gear" on the crane to accelerate up to the maximum speed while the load is moving. You select the appropriate gear for the next step in the procedure while the load is stationary and the crane brakes are on.