Civil Engineering Reference
In-Depth Information
Figure 7.1
Sliding pot bearing (Image: CCL Stressing Systems Ltd)
to the piston. The piston and the cylinder are attached to base plates that connect to
the concrete of the deck and pier respectively. Such a basic bearing allows rotation
but no displacement. Standard pot bearings can carry a limited amount of horizontal
load, usually about 10 per cent of the rated vertical load. Special pot bearings may be
manufactured where larger horizontal loads must be carried. Pot bearings are normally
sized such that the pressure on the concrete of the deck and pier does not exceed
20 MPa.
It is very important that the designer does not accept proprietary items such as
bearings uncritically. There is competition between manufacturers, and not all
bearings are made to the same standards. By studying the catalogues, one can deduce,
for instance, the working stress assumed on the rubber disc and on the concrete. The
author has had the experience of a bearing supplier being undercut by a competitor
who was simply rating his bearings more highly, and thus supplying a smaller product
for the same load. Also, mechanical standards differ; for instance the pot may be
simply welded to the base plate, or may be machined into it; corrosion protection
systems may not be to the same standard.
7.4.2 Guided and free sliding bearings
If the bearing is to allow free sliding in any direction, the top surface of the piston
base-plate is covered by a PTFE coating, and a further plate, equipped with a highly
polished stainless steel sheet slides over it. The sliding friction of such bearings depends
on the contact pressure and on the cleanliness of the sliding surface. In reality, it may
vary from 0.5 per cent up to 5 per cent.
If the movement of the bearing is to be guided, a keyway and corresponding key are
machined into the top two plates, or side guides are attached to the top plate.
7.4.3 The life of PTFE
The PTFE coating of sliding bearings wears with use, and it is not clear how long it
will last [2]. Once the PTFE starts to wear out, the friction coeffi cient will increase,
