Re: High floor and rotating trucks
  Tony Prescott

--- InTramsDownUnder@..., "Bob Pearce" <frerrick@...> wrote:
>

> Hi all,

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

> them?


Mick and Bob, the reason for the stub axles is of course to get the floor down low between the wheels. There are still some models with continuous axles but then the wheels have to be quite small to minimise the intrusion into the floor. If you have larger diameter wheels then you've got a problem getting the floor down because the axle is higher. I feel Bombardier and Alstom are addressing this by ramping the floor which means you get an "up and down" floor level (about 15 cm variation) as you walk through the tram. Probably this isn't a great problem but we'll see the reality when these trams come into service. Certainly the hub motor trams (Skoda and Variotram) are the only ones that can achieve a true 100% flat floor without seats over the bogies being shoved upwards.

Then of course you can't get the motors on the axles the old way because these would really bust into the floor level. So the motors go longitudinally on the outsides between the fore and aft wheels of the truck. That's still two motors but you need gearboxes and drive shafts (more things to go wrong). I'm generalising here - manufacturers have their own approaches to the issue.

The beauty of hub motors is that they can be readily lifted off for service and they can be electronically coordinated to drive the wheels at different rates both sides to go round curves, as well as skid mitigation etc, just like the ETC systems on motor vehicles nowadays. Now with the move to synchronous PM motors (Skoda) the gearboxes are dispensed with as well. (Alstom is also starting to use synchronous PM motors but being still located longitudinally one each side they are still using gearboxes.) So the cost of four motors rather than two tends to be levelled down a lot. Certainly there is no impact on the overall cost of such a tram compared with the other type. In fact in the case of the Skoda it is slightly cheaper.

I feel the lack of a continuous axle is not a problem as there is a very rigid structure between the wheels. Certainly the Skoda 15T has an exceptional ride quality as well as being gentle on the tracks so seems to have proved that a continuous axle is no longer necessary to maintain these qualities..

Bob, a good illustrated coverage of the evolution of the issue is here:

http://webak.upce.cz/~lata/konference/sbornik-clanky/79_86_zelingr_heptner.pdf

A translation is below at the end of this post.

cheers
Tony P
(but always open to alternative opinions - these things are only moved forward by debate and discussion)

Translation of Zelingr and Heptner paper

ROTATING BOGIES IN LOW FLOOR TRAMS
Miloš Zelingr, TomᚠHeptner

19th International Conference – recent problems in rail cars
21-22 September 2009, &#268;eská T&#345;ebová
1. Introduction
This article describes the relation between the concept of the tram car and the bogie, with focus on rotating bogies.
2. What is a rotating bogie?
Definition of rotating bogie from the text "Rail cars", author Prof. Nejepsa (1953): "A bogie is a low cart with two or more wheelsets, with wheelbase 1.8 to 3m (even more), carrying the bodywork in such a matter that it can rotate in relation to the vertical axis".
3. The bogie is a key component of a low floor tram
Low floor trams have most of the floor 350mm above the rail - that is, the height of the axle of a PCC tram, like the Tatra T3. Above the bogie the floor is usually a little higher. 100% low floor are considered trams with a floor height up to 450mm with ramps between sections with different floor heights. To achieve this the bogie had to change. The key was replacing the axle of the wheelset with a different type of wheel setting
3.1. Rotating bogies with wheels of small diameter
One of the variants is the use of wheels of small diameter. This solution was used in the 1980s and 1990s on trams like Be 4/8 (VeVey) or NGT 8D (Alstom LHB – see Fig 2 and Fig 3). There, these bogies with small wheels were placed beneath the middle low floor segment.
3.2. Fixed bogies
The corridor for passengers above the bogies with small wheels was very limited, usually only 500mm, and the seats had to be on an elevated platform. This, and the disadvantages of the small wheels, have led to the use of fixed bogies or partly rotating bogies [1 to 5 degrees – like the Škoda 14T]. Trams with these bogies form now the largest number of trams made in the last 20 years.

It is clear that the use of 600mm wheels on a rotating bogie would limit the space for passengers very much (Fig 4). Also the use of an axle 300mm above the rail was not possible. The best solution found was the replacement of a wheelset on an axle with two independent wheels on a welded girder (Fig 5) or on a pivoting beam.
This configuration was used on fixed bogies under the short segment in the middle of a three part articulated tram (Fig 6) under each of the three segments (Fig 7), or under odd segments of a articulated tram with odd bogie segments and even floating segments (Fig 8). The bogies of this concept are on Fig 9 and Fig 10 - Siemens bogie with welded girders and Bombardier bogie with pivoted beams.

The concept of the bogie is related to the system of drive – for example a longitudinal motor with a bevel gear, a transverse motor with straight gear or a hub motor with planetary gear.
3.3. Other solutions
There are other concepts like portal two wheel bogie on the [Austrian] Siemens ULF tram, or pivoted four wheel bogies with steered wheels on the Bombardier Cobra. They have not come into wider use.
4. Comeback of the rotating bogie
The low floor trams with fixed bogies have problems with the peaks of leading forces when entering a turn because of the small base area that initiates the rotation of the trams into the turn. Differences in the leading forces are on shown Fig 12. In spite of the economic success of the fixed bogie tram, the retreat to older concepts and new innovations has occurred.

Underneath the [high] floor segments are, again, rotating bogies with standard sized wheels (e.g. Bombardier, Fig 14). On articulated trams with [bogies] under the first and last [high-floor] segment, where the leading forces are a major problem, rotating bogies with small wheels are used again (e.g.. Alstom, Siemens – Fig 15; Fig 16).

Nowadays there are two concepts of driven rotating bogies for [100%] low floor trams that differ in concept.

The Citadis class X[04] (Fig 17) has two rotating bogies under the first and last segment and two bogies under the middle segment. The bogies (Fig 18) have two synchronous motors; each drives one wheelset on an axle through a bevel gear.

The Škoda 15T (Fig 19) has a rotating bogie with off-centre bearing underneath the first and last segment and a [rotating] Jacobs bogie under each joint (Fig 20). The wheels are driven individually by a synchronous motor without a gear through a short axle between the motor and the wheel.

Both of these trams have a floor height 350 mm above the rail, elevated [by about 100 mm] above the bogies with ramps connecting the two parts.
5. Conclusion
From the trends in low floor tram design it is clear that ways are sought to return to traditional concepts.

The rotating bogie is a key component of classic trams and it seems it will be used in low floor tram design even more.