Windshield Wiper B747-400

[NSEU]

Folks... I'm having trouble understanding the mechanics (and some of the electrics) for the 744 Windshield Wiper.

For each wiper, a DC motor drives a gearbox/converter which converts the motor's rotary motion into oscillating motion (i.e. there is no requirement to change the direction of the motor when the wiper is running). If the gearing is purely mechanical, then it follows that the angle of sweep will be fixed (about 70 degrees on the 744)

How then, does the wiper arm park itself several inches off the windscreen (outside the normal sweep range). As far as I know the motor does not mechanically disengage itself from the gearbox.

I read that when the switch is set to OFF, the electric motor reverses. This, presumably reverses the sweep of the arm. However, surely the range of sweep must still be bound by the mechanics of the gearbox/converter (70 degrees).

Neither the Maintenance Manual, wiring schematics nor training notes make any sense of this.

All theories welcome :)

Cheers.
NSEU

[Spanner Turner]

Mate, not sure if you had access to this section, but this is about the best explanation I can find in words.
Otherwise you'll be up for pulling one apart to satisfy your curiosity.
The key to the park position is the mechanical park switch which has to be activated to cut power to the motor.




Low Speed Operation
When the "LOW" setting is selected, relay "R1" is energised. Closing the switch, energises fields 1 and 2 in series. The motor then
starts to operate at approximately 160 strokes per minute.


High Speed Operation
Setting the switch to the "HIGH" position, energises both relays "R1" and "R2". With both relays energised, field 1 and field 2 are energised in parallel. The circuit is completed
and the motor operates at an approximate speed of 250 strokes per minute.



Park Operation
Setting the switch to the "OFF" position, will allow the relay contacts to return to their normal positions. However ,the wiper
motor will continue to run, until the wiper arm reaches the park
position. When both relays are open and the park switch is closed, the excitation to the motor is reversed. This causes the area swept by
the wiper to shift downward. The wiper blade goes off the lower edge of the windshield and the cam-operated park switch opens. This
de-energises the motor and releases the brake solenoid, applying the brake. This ensures that the motor will not coast and reclose the park switch.


:ok:
.

[NSEU]

When both relays are open and the park switch is closed, the excitation to the motor is reversed. This causes the area swept by
the wiper to shift downward.

Thanks, SpannerTurner :ok: This is the part I don't have in my manuals. However, how can simply reversing a motor cause a mechanical gearbox/converter (which produces oscillating motion) to change its area of sweep? For example, if you change the direction of pedalling on your bicycle, it doesn't cause the pedals to go any higher or lower (top and bottom) :confused:

There must be something pretty magical about this gearbox :}

Cheers
NSEU

P.S. I'll leave the gearbox dismantling to the experts, I think :ok:

[jettison valve]

NSEU, you got me thinking... :eek:

For the Rosemount wiper motors P/N 2313M-248-1 and -2, I found in CMM 30-40-01:
"Converter Assembly
The converter assembly reduces the speed of the rotary output of the motor assembly and converts it into an oscillating motion for driving the wiper arms and blades back and forth across the windshield.
During the OFF (Park) cycle, the motor reverses, which makes the converter
mechanism reverse direction. When this happens, a parking eccentric (working with the driven and driving cams and a wrap spring) changes the wiper pattern to allow the off-pattern parking of the wipers. "

and

"OPERATION
The pilot and copilot each have individual control over their motor-converter assembly by means of separate control switches that can be set to HI, LO, or OFF. The field coil of each electric motor is wound in two segments. When the control switch is set to HI, the two motor windings are connected in
parallel, which provides maximum motor revolutions. When the control switch is set to LO, the two motor windings are connected in series. In the OFF position, the motor will operate at low speed and simultaneously reverse its direction of rotation until a mechanism in the converter changes the wiper
pattern to park its wiper blade in its off-pattern position. When park position is reached, a trigger- actuated switch in the converter will remove all electrical power from the motor (see Figure 2). "

Hope this helps!

Cheers J. V.

[NSEU]

Great stuff, J.V.

I never thought to look for a CMM. I thought the converter might be more than a simple gearbox, otherwise it wouldn't work as advertised.

Thanks for your time. Much appreciated.

Cheers.
NSEU

[jettison valve]

NSEU,

No worries - I have appreciated your inputs in the past. And, as I said before: Your thoughts got me thinking. :ok:

I must admit that I havn^t fully understood yet how this thing works. On the output / wiper shaft there must be the trick with the cams, looking at the (single) figure in the CMM.
But I am afraid only by looking at the CMM, I won^t get behind the secret. Maybe I^ll run into a B747 wiper expert some time...

Happy Landings,
J.V.