Siemens SF7000 Bogie
Background
The new Thameslink stock will bring a number of innovations but key to the rolling stock will be weight and to get this element down much effort has been expended by Siemens on the SF7000 bogie which will be used on the fleet. The current SWT Desiro fleet uses the SF5000 bogie and travellers will appreciate the ride quality this provides.
So why replace the SF5000 – the response from Helmut Ritter, Head of Engineering, Bogies: “We needed the new bogie to be lighter than the old SF5000 – much lighter,” he commented. “The reduced weight would consume less energy, and also be kinder to the track resulting in lower track access charges for our customers.”
Bogie Construction
So the weight needs to come down and the welded bogie frame is inside rather than outside the wheels. It uses modern robotic welding technology. This design and construction saves a considerable amount of weight in itself as far less steel plate is involved in the construction simply by being narrower. Against this it reduces the space available and so makes it more difficult to fit the equipment.
The bolster normally rests on top of the bogie, and contains air reservoirs for the suspension and adds stiffness to the construction. It is also very heavy, so it has simply been removed! The air reservoirs have been moved up into the carriage body. Stiffness is achieved entirely from clever frame design. Finally the steel axle is normally heavy so to save weight a hollow design has been adopted and with inside bearings it is shorter too. Overall the unsprung mass is reduced which will lead to extended maintenance periods.
Brakes
Three types of braking have been incorporated into the design – but again with weight saving in mind they are not all bogie mounted. Over the last few years regenerative braking – essentially using the motor on the power bogie to return electricity to the supply acts as a brake on the train consuming kinetic energy. Sending electricity back reduces consumption and therefore cost and is the primary speed reduction system utilised.
Next up the power bogies are fitted with tread brakes which are normally considered noisy and inefficient but the use of organic brake pads enables other benefits to be derived as they are quiet and do not cause wheel damage and as they are applied to the treads they keep the contact surface clean, free from oil, leaf and other debris and therefore improve adhesion. The tread brakes had previously been utilised on the SF2100 bogie used in Shanghai. These will only be applied in emergency or in the final stages of bringing the train to halt.
Finally on the unpowered bogies there are two large ventilated disk brakes which give good stopping power and of course there is more space for them to placed inside the frame as there are no motors occupying the limited space. All three methods of braking are called upon together to bring a train to a halt.
Bogie overall
Having identified these key elements then the remaining parts of the bogie are designed around the weight saving elements already discussed. No stone has been left un-turned with small components being examined and redesigned. As an example yaw damper brackets which are placed outside the bogie and tend to receive damage from ballast and other debris moved by the airflow around the train have been redesigned with harder materials in thinner pieces and were then subject to stones being fired from catapults until a new design stronger than the original was achieved.
With the aim of good performance and low track wear the wheelbase on the motor bogie is 2.2m and 0.82m diameter wheels are used (when new wearing down to a minimum of 0.76m). Primary suspension uses a rubber layer spring with a secondary air spring. The wheelbase on the trailer bogie is 2.1m. The bogies can cope with higher vehicle payloads whilst keeping the same level of safety against derailment.
The motors are integrated into the bogies and each one delivers 235kW, so 470kw per bogie. For those who are not quite there in metric terms this equates to 630hp. These appear to be slightly lower powered than the motors fitted to the existing Desiro fleets. Oddly a different Siemens source quotes 3.3MW for the 8 car trains which is only 208kW per motor.
So having spent a lot of money what is the end result? Well the SF5000 weighed in a 9.3 tonnes and the new design is 6.3 tonnes for the motor bogie whilst the unpowered bogies show an even greater percentage reduction from 6.8 tonnes to 4.4 tonnes.
The bogies are being built at the Siemens plant in Graz (Austria) before being forwarded to Krefeld where the new trains are built. Graz has been building railway vehicles since 1854 and joined the Siemens family in full in 2001 becoming the global home of bogie development for the company since then.