Success in civil engineering with slip liners and timing belts
BRECOFLEX® BATK for Slip Liners
Plastic slip liner pipe with UV lights.
Sewers and pipes age and wear. After decades they become cracked, start to leak and become penetrated by roots. In 1971, the agricultural engineer Eric Wood had an innovative idea for replacing worn and damaged sewer pipes without having to dig up the roads for every defect: It consisted of a pipe-in-pipe (slip liner) system for rehabilitating sewer pipes without excavation being necessary. However, it took until the 90’s before the slip liners reached today’s high level of development. How is the plastic slip pipe inserted into the sewer pipe without excavation work, and what role do the timing belts play in this process?
Timo Singler, design engineer for winding machines at Brandenburger Liner GmbH & Co. KG in Landau, explains how the slip liner is inserted inside the sewer pipe: “It’s similar to putting a model ship inside a bottle. The slip liner is folded because it is ductile, as isn’t cured until it is in place. The slip liner – a glass-fibre reinforced plastic fabric pipe impregnated with a liquid thermosetting resin – is inserted into the sewer pipe, inflated with compressed air to fit the pipe and then cured.”
Once the slip liner is inflated with compressed air in the sewer pipe, it takes time to expand to make a close fit with the inside of the pipe. Only then is the resin cured with UV light. “With a speed of up to 2.0 m/min, it is the most effective method on the market,” Timo Singler says. Powerful UV lights strung together in several segments serve as a light source. The chain of lights is equipped at the front with a camera for monitoring the process.
Brandenburger from Landau in the Pfalz developed this curing technology ready for series production in 1993 in cooperation with BASF. In just over 20 years, they produced seven million metres of liners with their patented winding technique.
Although the company specialises in composite materials for civil engineering and the pipe-in-pipe system, it also operates its own mechanical engineering department for building special-purpose machines and develops its own production plants in-house.
Production machines developed in-house
Brandenburger produces slip liners from DN150 to DN1600 on four winding machine lines and supplies civil engineering companies throughout the world. The new winding machine line shown here covers a diameter range of 800 to 1600 mm. “We produce liners on this machine to lengths over 300 m. This can amount to over 35 tonnes for a delivery with a value of more than 100,000 euros”, the Brandenburger design engineer explains.
The manufacturing principle of the slip film is always the same: A thin slip film tube is held in shape on the inside by an expandable mandrel – referred to as the “tongue” – and transported by integrated timing belt drives in the longitudinal direction.
A winding form rotates with the resin-impregnated glass-fibre reinforced plastic fabric rolls around the fixed tongue with the slip film tube until the desired wall thickness is reached. The flat end of the tongue ensures that the slip liner is folded together and can then be wrapped with a protective film and sealed longitudinally.
The winding machines are also an in-house product
The dimensions of the new machine lines are formidable. The 2.5 tonne tongue is flanged onto a tubular steel structure with 4 bolts to take the tube film supply. The timing belts are driven via a drive shaft running through the steel structure and a centrally positioned worm gearing. The worm gear drives each of the eight timing belt drives by means of one spur wheel each.
The tongue can be precisely adjusted in diameter by means of radially arranged rack and pinion gears until the tube film is tensioned. The deflection rollers are so ingeniously arranged that the wrap-around length for the timing belts always remains constant and the belt drive doesn’t have to be readjusted.
The dimensions of the polyurethane timing belts of the type BRECOFLEX® BAT are also huge: The endless timing belts have a circumference of more than 10 m. “This is a challenge for the production process and is possible only with the patented manufacturing process of Mulco’s partner, BRECO Antriebstechnik from Porta Westfalica. The polyurethane is completely extruded around the tension members and then sanded”, engineer René Preßler from Mulco’s sales partner, Hilger u. Kern, explains.
Transport Timing belts guide and transport
The drive technology specialist continues: “We normally recommend to our customers not to tension a coated timing belt via the back of the belt. However, as the diameter adjustment of the tongue with its deflection rollers makes this necessary, we recommended precisely adapting the profile of the timing belt backing to that of the deflection rollers.” Breco Antriebstechnik manufactured a special sanding disk according to the design of the deflection rollers. The high-quality synthetic rubber, linatrile, is used as a backing. This material is particularly notch ductile. In this context, this describes the resistance of the material against a penetrating object. “It feels similar to a rubber,” Timo Singler adds.
The Brandenburger design engineers chose the BAT timing belt profile with arc-shaped teeth because it is self-centring and can do without flanged pulleys. However, the BAT timing belt has a fixed running direction, i.e. it can completely fulfil its function in only one rotational direction. “To date, all machine lines at Brandenburger have only one preferred direction, including the new machine line up to 1600 mm diameter”, engineer René Preßler explains.
However, in trials it was discovered that the large supply of film tube can also be elegantly automatically transported and pushed on with the tongue in the reverse direction. In the worst case scenario – when the tongue is moved in the reverse direction without film – there was a risk of the timing belt coming off a pulley.
Self-centring BAT and BATK timing belts
For this, there is the BRECOFLEX® BATK polyurethane timing belt with arc-shaped tooth profile and a guide wedge in the centre that runs in the matching groove of the pulley. It is suitable for both rotational directions. However the BATK profile is available only at a width of 32 mm or more. As the new machine at Brandenburger was already equipped with 25 mm wide timing belts, a change to 32 mm timing belts would have led to disproportionally high conversion costs.
“René Preßler found an economical, quick and very practical solution for us”, Timo Singler says, and adds: “We use flanged pulleys in the area in which the belt could otherwise drop off. This means that we are replacing only a few pulleys – at a fraction of the cost of changing to 32 mm wide BATK timing belts. For the next revision of the design we will use a BATK32.”