[ Ref. Jeff Mason in 160504 MMDigest --

By coincidence, for the first time in some years I happen to be
rebuilding several Duo-Art rotary pumps.  It seems likely that
Duo-Art and some Ampico and Welte rotary pumps were bought in from 
the same manufacturer in USA; in many cases their castings are
identical, whether in US, British or German instruments.  It would
be good to discover what company this was and what the printed
VOVTOR and VOVGOA labels signify.

As far as I know there are two series of Duo-Art rotary pumps:
those with adjustable ball races for the con-rods and those with
non-adjustable standard deep-groove ball bearings.  I suspect the
deep-groove type supplanted the adjustable type here in Britain
sometime around 1926.  Adjustable bearings probably seemed best to
secure silent running, but I guess it was found in practice that
deep-groove bearings are quiet until badly worn.

Removing original bearings is not in always straightforward as most
are tightly swaged in and essential access to the back of the outer
races to press them out requires some ingenuity.  Probably only the two
main crankshaft bearings were ever intended to be removed and replaced.

Curiously, original non-adjustable deep groove bearings seem invariably
to be metric, even though other threads and dimensions in the pumps are
in inches.  I suppose this is because they are Swedish SKF/Norma.  They
are still standard sizes and easily obtainable here in the UK.  For
example:

  crankshaft: 2 x L17 Magneto;
  crank centre: 1 x 6200 (some are self-aligning versions);
  con-rods: 8 x 608.

But the adjustable bearings are a problem.  If the races are not
worn or spalled, installing new balls and modern lubricant may work.
It's a mistake to pre-load these bearings appreciably -- finger-tighten
them only till there's just no play and lock them without altering the
setting.

However, all three pin, cone and cup races are frequently worn
irregularly and can't be turned to present new surfaces, and so
continue to knock.  Replacement races are not available, so standard
deep-groove bearings are the best option.  This requires pressing out
the cups and shouldering the pins to accommodate suitable bearings.

I had all eight pins in one pump turned down appropriately but we
discovered the case hardening of the races had embrittled them and
several subsequently fractured across completely.  Fortunately, better
replacements are not difficult to make.  (Bolts, sleeves, washers and
locknuts could be used.)

Surprisingly good quality Chinese bearings are available very
cheaply on eBay which are perfectly serviceable for the duty required,
especially the eight con-rod bearings which hardly rotate.  Fixing
replacements with medium strength liquid bearing fit[*] rather than by
swaging is safe.  The original crank and con-rod bearings had no seals
and usually developed an abrasive mixture of grease and paper dust.
Today it's best to fit rubber-sealed pre-lubed bearings wherever
possible.

It's vital that the two new crankshaft bearings are pressed completely
home, and there is effectively no pre-load on them when the flywheel is
fully threaded on, or they will wear quickly.  Deep-groove races,
especially modern ones, can take a small degree of angular misalignment
without undue wear.  However, it's best to bolt the stirrups onto the
bellows last of all, avoiding any side load on the con-rod bearings.
A 1.5 mm (1/16-inch) thick shim of neoprene inserted under each stirrup
in place of any felt, and the stirrup nuts just nipped up and
Loctited, will allow a small amount of silent flex.

These pumps can be rebuilt to work reliably and silently, but how much
better were the designs using rubber straps without bearings...

Patrick Handscombe
Wivenhoe, Essex, UK

 [ * E.g., Loctite 641 controlled-strength retaining compound
 [ http://na.henkel-adhesives.com/product-search-1554.htm?nodeid=8797714186241 
 [ -- Robbie