What is "gearing"?

[straller]

Hello everyone,

I don't know much about AC traction, but I have heard the term "gearing" being thrown around a lot e.g "The motors have been re-geared from 100mph to 75mph".

What does that mean? I assume it has nothing to do with actual mechanical gears...

 

[samuelmorris]

Motors do not drive the wheels 1:1 - the traction motor spins far more times per second than the wheels, as the wheels have a much larger diameter. An electric motor will only operate safely at up to a certain speed. By changing the ratio between the motor speed and the wheel speed, the maximum speed of the motor will provide a higher or lower maximum speed of the train. Regearing the motor down to a lower maximum speed will, in the same way as other transmission methods, improve acceleration, useful for improving timetables in areas where a high top speed is not necessary or would be unattainable (e.g. short distances between stations or a low line speed limit).

 

[rebmcr]

Motors do not drive the wheels 1:1 - the traction motor spins far more times per second than the wheels, as the wheels have a much larger diameter. An electric motor will only operate safely at up to a certain speed. By changing the ratio between the motor speed and the wheel speed, the maximum speed of the motor will provide a higher or lower maximum speed of the train. Regearing the motor down to a lower maximum speed will, in the same way as other transmission methods, improve acceleration, useful for improving timetables in areas where a high top speed is not necessary or would be unattainable (e.g. short distances between stations or a low line speed limit).

"Yes, it actually is mechanical." would have sufficed.

 

[yorkie]

For background info on how traction motors transfer power to the wheels, this video from the documentary Running to Time about the introduction of Class 91 locomotives, will no doubt be of interest.

Running to time (part 2 of 4)

[youtube]uwm4P-wKqgQ[/youtube]

"[Class 91]'s traction motors are suspended under the body, but within the bogie frame.

A short cardan - or drive - shaft runs into a transfer gearbox, again mounted on the bogie.

Two bevel gears take the power through a right angle.

A hollow quill shaft connects the gearbox to the wheelset with flexible rubber mountings.

The axel of the wheelset runs right through the hollow gearbox and quill shaft.

At the end of each motor is a disc brake. In this way, only the wheelset remains unsprung mass.

[Class 91] has 2 bogies with a traction motor for each of the 4 wheelsets...."

--- old post above --- --- new post below ---

"Yes, it actually is mechanical." would have sufficed.

Well, yes, but I disagree that would suffice.

 

[samuelmorris]

I thought a 91 was generally considered an unusual case?

 

[yorkie]

I thought a 91 was generally considered an unusual case?

Quite possibly, what's unusual about it?

 

[samuelmorris]

The use of the driveshaft and the transfer gearbox - I was under the impression that this was an unusual modification taken to reduce unsprung weight specifically for the 91. Is this not the case?

 

[straller]

Thanks for the replies!

 

[edwin_m]

Most electric traction has the motors mounted onto the bogie and driving the axle via some gears. However this results in too much "unsprung mass" for high speed units, which can damage the track. Hence the 91s have the motor within the bogie but actually attached to the body and driving via a flexible drive. Pendolinos and Voyagers do something similar but put the motors between the bogies.

 

[samuelmorris]

That I didn't know (about the Pendolinos and Voyagers) but makes sense now that I think about it.

 

[Western Lord]

The use of the driveshaft and the transfer gearbox - I was under the impression that this was an unusual modification taken to reduce unsprung weight specifically for the 91. Is this not the case?

Nothing unusual about the Class 91. You can either have axle hung, nose suspended traction motors (which increase unsprung weight) or body or bogie mounted traction motors with cardan shaft drive to the axles (flexible drive). The first AC electrics (AL1-5, or Classes 81-85) had flexible drive. The AL6 (Class 86) reverted to axle hung motors and the track received a serious pounding as a result, so Class 87 and later went back to flexible drive. All HSTs are flexible drive. Of course, the Western Region diesel hydraulics were also flexible drive.

 

[edwin_m]

Nothing unusual about the Class 91. You can either have axle hung, nose suspended traction motors (which increase unsprung weight) or body or bogie mounted traction motors with cardan shaft drive to the axles (flexible drive).

The difference is that in the 91, Voyagers and Pendolinos they are body mounted, but every other UK class with flexible drive has/had bogie-mounted motors.

 

[snowball]

I think some old classes (maybe class 86??) had "nose-suspended, axle-hung" traction motors, which I think means that one side of the traction motor has not even primary suspension but can sort of rotate about the axle, while the other side is supported by the bogie.

These are even worse for the track than fully bogie-mounted traction motors.

(Primary suspension = springs between axle and bogie;
Secondary suspension = springs between bogie and vehicle frame)

 

[straller]

I think some old classes (maybe class 86??) had "nose-suspended, axle-hung" traction motors, which I think means that one side of the traction motor has not even primary suspension but can sort of rotate about the axle, while the other side is supported by the bogie.

These are even worse for the track than fully bogie-mounted traction motors.

(Primary suspension = springs between axle and bogie;
Secondary suspension = springs between bogie and vehicle frame)

Heck, even 455s have them.

 

[cjmillsnun]

The difference is that in the 91, Voyagers and Pendolinos they are body mounted, but every other UK class with flexible drive has/had bogie-mounted motors.

EDIT. I am wrong... post content removed.