On 18/06/11 01:56, Bob Pearce wrote:
> Hi all,

> Which company invented the stub axle truck and what was the reasoning behind

> them?

Talgo has been using stub axles (on which the wheel can move for gauge
changing) for many many years on heavy rail. The Talgo 1 dates from
1942. So the idea is not new. (Guage changing versions are from the 2nd
half of the '60s)

But Talgo's have a very low centre of gravity, and have short
articulated cars. The trucks are steered into the curves by the body
articulations.

> Was it because the motor could sit on the actual stub axle, and drive

> one axle only,

No, to have no axle at all so there was nothing to stop a 100% low floor
through the entire car.

> It seems to me that if a motor was needed for each wheel,

> then the number of motors per truck has doubled and I don't see the cost

> advantage of that. I gather the size of the motor is quite small compared

> to Met Vic or GE etc type motors as used on the W's.

Smaller and cheaper.

Mostly hub motors haven't been used (the planetary gearboxes needed are
expensive and fiddly). Sydney's Variotram and the much discussed Ŝkoda
15T use hub motors. One of Ŝkoda's advances appears to be elimination of
the gearbox. The motors are direct drive.

I believe Alstom went for motors laid on their side along the
outside/top of the truck, and Siemens went for motors that hide up in
the body, hence the large intrusions into the passenger space.

If the AC motors are in lock-step you can sort of simulate a fixed axle
configuration, as wheels are not free to turn at their own speed, but
their speeds are fixed by the frequency feed to the motors, which would
be all the same. Despite the frequency lock, I would bet that in curves
the motors 'slip'. (Turn slower than the drive frequency would dictate,
which would eventually result in a loss of torque).

Alstom's latest low floor motor truck actually does have a mechanical
coupling across the wheel set, but not a conventional axle - a motor on
the outside of the truck frame drives one wheel on that side, and there
is a gearbox arrangement that connects to a low level trans-axle that is
geared to the wheel on the opposite side. The arrangement is reverse for
the other 'axle'. So the truck now has 4 driven wheels but two (larger)
motors.

> This has probably been discussed before but now I am getting more curious

> about it all, given the talk of new trams for Melbourne, etc.


Not sure what Melbourne is getting, but it appears the Gold Coast are
getting Flexity 2s. One of the listed features for the Flexity to is
'Flexx Urban 3000 bogie which allows the LRV to run on conventional
wheelsets'. However it's not a rotating truck design, so while the
proper wheel sets should allow it to track better and not 'hunt' as
badly as the stub axle designs, it's still going to hit the track hard
when curving.

Does the 15T have real axles ?, the Wikipedia article says that one
traction pack drives 4 motors on one truck. That would imply no axles.
It will be relying on the motor control system to stop the truck from
hunting.