What makes desert buggies indestructible
The main business of STERN Automation & Maschinenbau in Berge (Nauen) in Brandenburg is the development, production and sale of CNC cutting machines. The standard range encompasses three machine sizes for small, medium and large format panels. Company owner and passionate model maker Thorsten Stern also uses his machines for his own model construction. Carbon fibre reinforced plastic parts and aluminium parts for boats and buggies are produced on the in-house designed machines.
"It started as a hobby but thanks to my sons it has developed into an interesting business model," Thorsten Stern explains, not without pride. Today the family business supplies model construction components to countries as far away as the USA and New Zealand under the label "Stern-CNC Modelbaukomponenten". The remote controlled buggies weighing just under 10 kg are about 800 mm long, and are driven by a 23 cc two-stroke cycle engine with about 3 horsepower. That is easily enough for them to reach a speed of 60 km/h. Specialists use special fuel and tuned ignition timing to achieve double the speed. Torsten Stern reminisces: "About six years ago we started replacing the wear-prone, open and unlubricated spur wheel gears with timing belts. In those days that was completely new territory. The toothed gearing of the buggies often lasted only one season; the version made of plastic sometimes only a few races. We wanted to improve this. Low weight and a small mass moment of inertia are particularly important because fast acceleration is what the buggies are all about."
Reversal of the rotational direction with timing belts
To test the market for complete conversion kits from toothed gearing to timing belt gearing, Torsten Stern offered a first kit on an auction site as a "buy it now" product. "The kit was sold in six minutes," he says. Most buggies have gears with three gearwheels so that the drive and output rotational direction are identical and they can be replaced with a simple timing belt drive. The company has been using these drives successfully for many years. However, at first he thought it would be difficult to convert the desert buggy Baja.
This is equipped with a simple set of spur gears with two gears for changing the rotational direction of the drive. "My son had the great idea of using a double-sided timing belt to allow the rotational direction to be changed when using a belt drive," Thorsten Stern explains. By using a double-sided timing belt, the belt can be guided between the two shafts along the inside of the small pulley, changing the rotational direction.
This drive required much more development work, which was why the three model makers sought professional assistance from the Mulco member and specialist for timing belt technology, Wilhelm Herm. Müller GmbH & Co. KG, Garbsen. André Schmidt, engineer and customer consultant, has been advising industry customers from the Müller branch in Leipzig for more than 20 years on the subject of polyurethane timing belts. He explains: "Stern had already manufactured prototypes so that our job was to produce drawings for the pulleys and give advice on the permissible manufacturing tolerances to ensure the correct belt tension. A fixed centre-to-centre distance was chosen for this drive to exclude possible installation mistakes by the customer and damage to the bearings and belts."
Torsten Stern designed an additional outer support plate for the two pulleys to reduce the load on the bearings and to avoid skewing of the pulleys. The support plate is made of carbon fibre for reasons of weight. "Due to the small pulley diameter and the large wrap-around, we chose a flexible CONTI® SYNCHROFLEX polyurethane timing belt with a 5 mm pitch. The 15 mm wide belt T5-DL (DL double lip) transmits the 3 horsepower rated load of the petrol engine without any problems and there have not been a single failed drive sto date," says André Schmidt.
Customized timing belt solutions in small quantities
The timing belt drive is protected from sand and stones by a specially designed cover. "Some customers like the appearance of the drive so much that they leave off the important cover," Torsten Stern says with a grin. Without the cover, stones can get into the drive and tear the timing belt. "We simulated this on our test stand and discovered that in the worst case scenario the steel cord of the torn belt can wrap around the rotating shaft," the resourceful inventor explains. André Schmidt from Wilhelm Herm. Müller suggested using tension members made of aramid as a customised solution: "Aramid doesn't develop such high tensile forces and tears when overloaded. This avoids additional damage due to the tension member wrapping around the shaft."
Three transmissions with one belt drive
The inventors from Nauen have achieved an ingenious design with the two guide pulleys: the driven pulley and the two guide pulleys with different numbers of teeth can be interchanged. By interchanging the pulleys it is possible to operate the buggies with three different transmissions depending on the race course and selected tyre size. The original has only one set of gears with a fixed transmission. That is real added value.
Three further variants as an alternative to the standard tension members made of steel are available for the CONTI® SYNCHROFLEX polyurethane timing belts: Aramid tension members, tension members made of VA stainless steel and E tension members made of steel with specially thin individual wires.
In the E tension member, the cross-section of the tension member is distributed to a larger number of thinner individual wires, therefore the bending fatigues are markedly lower in the individual wires. The advantage of the E tension members is a higher flexibility. This is especially important when smaller mounting dimensions for pulleys and tension rollers are required. It is possible to reduce the minimum number of teeth and/or minimum diameter of the pulleys by up to 30% compared with standard tension members. Timing belts with E tension members are recommended for multi-shaft drives with frequent bends.