Sailplane with a skid
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tost shoe brake wheel sailplane brakes kobold wheel

Editors Note:  Braking systems have evolved well past the skid.  Effective sailplane braking is important to every flight.  Here is an article from Tost on brake maintenance adjustment.  Tost produces some of the most effective sailplane braking systems available today.

By Barbara Dorflein
 

Tost Shoe Brake Wheel

Shoe brake wheel advantages are simple construction, reduced space requirements, lower weight, lower force on the brake lever because of the servo action, and of course, the lower price compared with a disk brake wheel.

Shoe brake wheels are fitted with non-servo brakes: as the brake wrench turns, the primary shoe is pressed outwards in the direction of rotation of the shoe, the secondary shoe in the opposite direction. The clutching effect on the primary shoe increases the contact pressure and results in a significantly higher braking action than for the secondary shoe. This is the reason why the brake pads wear differently.

Construction

Tost shoe brake wheels such as the 4" Kobold pictured below are constructed in two parts. They are manufactured from an LM alloy by gravity die casting. Special heat treatment increases the strength and improves the alternating flexural strength to achieve optimum fatigue strength.

TOST Kobold Wheel

The rim flange with integrated segment chambers serves as a crumple zone. In the event of over-loading, load peaks are dissipated but the emergency rolling characteristics remain unaffected. In the brake-side wheel, half a pearlitic brake ring is cast in to improve the coefficient of friction.

The ball bearings with sealing rings are maintenance-free. Tension of the bearings is prevented through the use of inner spacer bushes.

The brake-/anchor plate is equipped with a brake lining set with spring(s), brake lever, and brake cam and (partly) with anchor bolt, (see schematic drawing below).

Tost Shoe Brake Wheel

1 - Anchor (brake) plate complete
2,3 - Set Brake linings + Springs
5,6,7,8 - Set brake lever and brake cam
9 - Anchor bolt
 

Installation and Operation

  • The installation of all Tost wheels is done without axial tension. The hollow axle is simply pushed through the ball bearings. Note: The inner spacer bush should not be freely movable axially. The fault could be a pinched tube after tire and tube exchange or the wheel halves are not assembled properly.
  • The two rim halves are connected with high-tensile DIN 912 M6 bolts (10.9 strength or higher). Tighten bolts with a calibrated torque wrench to 9-10 NM
  • Warning: Do not use bolts of lower strength. Always use a torque wrench. Most failures are the result of too high tightening torques
  • Make sure the brake cable is laid correctly: as short and direct as possible, but without too tight bends. Use a Bowden cable (recommendation: cable dia. 2 to 3 mm) with a set screw, for exact clearance adjustment

Maintenance

  • Re-adjust the (Bowden) brake cable depending on wear of the brake linings.
  • Adjust the brake lever at regular intervals depending on wear of the brake linings. Brake lever and brake cam have both a fine toothing, for precise adjustment. When the brake lever is initially set or subsequently adjusted, the nut must be tightened with the stipulated torque:
  1. M8x1 DIN 936 =  18 to 22 Nm
  2. M 6    DIN 934 =     6 to 8  Nm

Warning: If the nut is not tightened properly, the notch-toothed connection can slip and the toothing can be destroyed. Always exchange the 2-part set brake lever and brake cam, if one side of the toothing is damaged.

  • If brake linings are glazed after prolonged or excessive use, the brake effect is significantly reduced. Clean glazed or scorched lining surface, as well as the brake ring, with methylated spirits and emery paper.
  • Replace brake shoes if the lining thickness is less than 1.5 mm at any point. Warning: Continued use of brake shoes with worn-out lining can damage the wheel. You are strongly advised not to reline brake shoes.
  • Replacement brake shoe sets are turned in pairs to the right diameter. Never replace a single brake shoe. Brake shoe sets are color-coded and must be fixed to the brake anchor plate in that way that the color mark is visible.
  • New brake linings must be run in. The first 20 to 30 landings should therefore be carried out with careful braking.

Banner photo by Manfred Franke

Barbara Dorflein    Barbara Dorflein is one of four generations since her grandfather, Richard Tost founded the company in 1945.  When German gliding was allowed again in 1951 safety was an issue.  There was a strong need for safe aircraft parts.  Shortly after, a product line of gliding safety equipment was developed and the company was renamed Tost Flugzeuggerätebau München.  This set the course for the future. Since then, the family company has concentrated on the manufacture and sale of equipment that makes the operation of gliders, sport aircraft, and helicopters safer.