Low-friction bearing of the rotary axes in the automatic closure of a convertible top

Profile

• What was needed: iglidur plain bearings
• Requirements: Due to the only one-sided drive of the canopy mechanism and the low spring force in the open position, a low-friction bearing of the rotary axes is necessary; the bearing should also allow a fixed axial clearance, which, however, should not be too large
• Industry: Automotive industry
• Success for the customer: With iglidur plain bearings, the customer found a very cost-effective solution; the maximum axial clearance could be reduced from 1.5 mm to 0.5 mm by the use of the calibratable flanged bushing

Problem

The application presented here  for a plastic bearing concerns an automatic closure for a convertible top (soft top) for locking the front roof bow to the vehicle windscreen.
The convertible top is moved by means of hydraulics over almost the entire pivoting range before the electric motor-driven automatic closure closes the fabric top completely. A lateral auxiliary lever and a four-bar linkage driven from one side act at the front. The four-bar linkage is opened by a roller in the base plate of the windscreen panel after a slide has released the lock. The necessary movement for this is generated by the side lever and the canopy hydraulics. A helical spring supports it in its gearbox action.
To ensure that the arrester hook (coupling of the four-bar linkage) not only releases the roller of the base plate, but also actually reaches the structural end position, a low-friction bearing of the rotary axes is indispensable due to the only one-sided drive and the low spring force in the open position.
The bearing of the link lever is designed as a fixed bearing, the bearing of the delicate control lever as a floating bearing. In fact, however, the axial clearance in the fixed bearing - due to the manufacturing tolerances in the cast parts - can be up to 1.5 mm, so as not to jam under any circumstances if the tolerance position is the reverse. However, the following problem arises due to the clearance: if the arrester hook deviates too far from the desired position, it does not enter the windscreen, which allows only little room for movement. The remaining room for movement of the floating bearing on the control lever can thus be restricted in such a way that clamping  is possible on the rotary axes of control lever/arrester hook or drive lever/arrester hook.

Solution

For the manufacturing company EDSCHA Cabrio-Dachsysteme GmbH , the following possible solutions were offered to reduce the high reject rate:
A change of tooling on a casting, a gluing in of the centrally located flange bushing when installing in the link lever or a calibratable design of the centrally located flange bushing. The drive lever is calibrated to a gauge block with tolerance ±0.1 mm when installing the flange bushings.  
While option 1 involves high modification costs for the tool, for an additional tool for clamping and for milling over each link lever, option 2 would have required the necessary coating of the flange bushings with acrylate adhesive, which would have required an additional work step that is also hazardous to health.
The decision was therefore made in favour of the 3rd option, which offers significant advantages in application:
While the tool for a new iglidur plain bearing could be provided by us at low cost, only a simple modification was required on the assembly tool for the flange bushings, which also kept the costs manageable. In addition, a possible spring effect between the demoulding slope of the support part and the exposed flange of the bushing (wall thickness at the flange partially reduced to 0.5 mm) helps to reduce the maximum achievable clearance by a further 0.1 mm. 
The maximum axial clearance could be reduced from 1.5 mm to 0.5 mm by using the calibratable flange bushing.