Even with the achievement of a flexible auxiliary support that will
allow changes of curvature (although partially restrained) to occur in a
panel, the whole object of the exercise will be defeated if the panel’s sup-
porting edge gets stuck and cannot move smoothly in the frame rabbet.
With lightweight panels it has been common practice to use Teflon/PTFE
pressure-sensitive adhesive tape to line the tray or frame rabbet, thus
reducing friction against the load-bearing edge of the panel. With large,
heavy panels, the reduction in frictional resistance achieved by Teflon tape
may only be sufficient to prevent total jamming. Movement of the panel’s
bottom edge is still likely to be erratic, however, with sudden jumps occur-
ring only when the warping stresses build up in the panel and exceed the
frictional resistance imposed by its weight. Also, it is not uncommon to
find environmentally responsive panels that have warped away from a slip
profile and have become wedged at the back of the frame rabbet.
Asolution to this problem ofreducing friction, found suitable for
the Keirincx-Savery panel, was simply to mount the bottom supporting
edge of the lattice on bearings. Several bearing designs were investigated.
Among them, linear slide bearings were found to be available with
coefficients of friction as low as 0.003 (i.e., a force of 3 gm will move a
1 kg load on a horizontal surface). These bearings are high-specification
devices for engineering applications and as a result are relatively expensive.
For the Keirincx-Savery panel painting, however, the type chosen
were simple bearings known as Ball units that were found to work
extremely well and are being considered for use on some even larger pan-
els. Apossible disadvantage of Ball units is that the minimum dimension
below the panel needed to accommodate them is 20 mm, whereas with
linear slide bearings it can be as little as 8 mm. Fortunately, the Keirincx-
Savery frame was substantial enough for 20 mm deep recesses to be cut
for the bearing to run in. Two Ball units were used, giving a combined
specified load-bearing capacity of 50 kg. Polished 18-gauge stainless steel
blanks were placed in the recesses as a running surface for the bearings.
If adequate depth had not been available in the frame, then the thinner,
more expensive type of linear slide bearing would have been considered
(Figs. 16, 17).
Since completion of the restorations,^12 the Keirincx-Savery
panel/support assembly, mounted in its frame, has been monitored at theT D F A A S 397Figure 16
Detail of the vertical support bearing.
Figure 17
The support bearing seen from below.