When Doug Bullock discussed the fire which seemed to start in John
Ravert's Duo-Art piano (MMD 12.12.19), he touched on some very
important issues.  I would like to offer more information which may
bring more understanding to the subject.

First, it is important to realize that the motors used in most American
built reproducing pianos lack any kind of built in thermal protection.
Most modern motors contain this protection.  Having thermal protection
means that a motor which becomes too hot will trip the circuit breaker,
and so the power to the windings is shut off.  That trip should occur
before the motor catches on fire.

Even a properly rebuilt motor, as described by Doug, may still lack
thermal protection.  This is why the second point I wish to make is so
important.  That is, it is not the "motor" that makes Duo-Art fires,
but rather the system in most cases.  Several conditions that can occur
within the Duo-Art system can result in motor overload, which will
result in fire if nothing corrects those conditions.

The spill valve system within the Duo-Art lacks protection to save the
motor from overload in every situation.  The spill valve is controlled
by collapse of either expression accordion reaching about level 10.
As an example, placing the tempo to zero when level 15 is set and few
notes are being played, will cause a tight system to create a stack
pressure well in excess of 100".  This will cause the motor to stall
or slow down enough to cause the start switch to reengage the start
windings.

Left uncorrected, the motor will soon overheat and the windings will
short-circuit and catch fire.  This was my personal experience one day
when I placed the Tempo control at zero to answer the telephone.  The
piano was not damaged as I removed the smoking motor immediately.

Aeolian partially corrected this problem in the 1930s by placing a
pneumatic trip for the electric shut off switch on the Tempo linkage.
If Tempo zero is set, the current is shut off to the motor and the
hazardous condition is avoided.  Still, the system is not foolproof
as it requires linkages in very precise regulation and it is easily
defeated by someone not understanding the hazard.  This modification is
found in the Duo-Art system that has the above the keyed controls and
around the key frame ends tubing.

Other systems (Ampico and Welte) have spill valves that function
regardless of control positions.  Those spill valves are usually
directly on the pump, and relieve excess stress on the motor in all
situations.  It is for this reason, that the Duo-Art has the reputation
for catching on fire.  The competing systems are less likely to
overload the motor.

All systems lack overheat protection, but it is the Duo-Art that is
most likely to overload the motor.  Situations that cause motor
overload are:

1.  Drive chain comes off the sprocket.
2.  Reroll fails (transmission fails to fully shift)
3.  Repeat fails  (transmission fails to fully shift)
4.  Transmission comes out of gear during play

Or any other situation that can cause the roll to stop moving.

Poorly set spill valves can make any system dangerous.

Never leave any operating mechanical machine unattended.  Don't leave
the room during rewind until the motor stops.

Bob Taylor
Missouri