Friday, 30 March 2018

1964 Cessna Altimeter disassembly, repair and reassembly

Aneroid Altimeter re-assembly.  
These photographs were taken after disassembly and repair, so read in reverse for disassembly. you really do need to read it from the end first.   It is fine, delicate work and even the slightest bump or excess torque can wreck the altimeter.  I started this venture because the altimeter fell off of the shelf onto a cement floor and the faceplate broke, and it stopped working.  There was nothing to lose, so i carefully started the process one evening.  
Looking at the FRONT of the bellows cage with the elevation hand(20s of feet  division) connecting to the very small central pin.  The gear surrounding this pin drives the thousands indicating hand.  

rear view showing bellows, spring bellows cage, and tension arm from bellows leading up to Torque arm  (I am making up names as I could not find detail )  

close up of torque arm, and thrust screw that holds the arm in place.  The end of the thrust screw is rounded, with a corresponding cavity hollowed out in the end of the thrust arm. 

The front, or other, end of the torque arm has a tension spring, counter weight and connections to another counterweight and tension spring, not detailed here. 

You can start to see how the expansion and contraction of the bellows is converted into the rotation of the elevation hands. this is a view with the tension rod from the bellows leading to the torque arm.  The springs on the torque arm push the the lever on the arm down agains the tension arm tot he bellows.  As the bellows expand, they let the torque arm "rotate" ever so slightly, counter clockwise in this photo

A wider view, with the thrust screw in focus.  

centre pin supports the "feet" hand.  The gear surrounding it drives the thousands hand

it is a little hard to put it all together, but this is the gear that drives the centre pin that holds the Feet indicator hand.    The centre shaft of this gear, has a much smaller gear the is driven by a 1/8th gear (just a portion of an entire gear, about 1/8th of a diameter) connected directly to the Torque Arm.  As it moves it drives the Feet hand through the gears. 

Close up of the Torque Arm gear (large and below in the image) and the conversion gear, (smaller gear) which drives the feet indicator hand, and also the thousands hand. 
Torque Arm being held in position by Thrust Screw.  

Tension arm from Bellows is connected to torque arm via a small piece of wire.  

The altitude indicating hands

cleaning old grease off of the front of the altimeter works cage

The back of the faceplate sits against the front of the altimeter works cage.  Three machined surfaces  keep  it square.  The Thousands and Tens of Thousands hands gear's are visible behind the support structure.

Cleaned up, buffed and shined with diamond cloth

The empty can, with the large bent, thrust spring along the outer ledge of can.  Altimeter adjustment screw visible upper left. When removing the altimeter works cage, gently shake out of the can into your hand.

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Same image without the labels

Inserting the face plate into can.  Obviously you have to be very careful to align with Feet hands centre pin.  When removing, lift opposite the adjusting gears, and gently shake forward.  

You have to put the adjustment gear side into the can first, then gently lower onto the altimeter cage and centre pin with gears.  There is an alignment pin, two photos up.

It took a couple of tries to get the thousands hand correctly oriented.  
Snap ring holds face plate assembly to altimeter movement cage.  Note alignment pin, on the right.
Snapring in place.  When removing use two small screwdrivers, ground to pins, to remove.

There are two of these very small black painted brass pins that hold the face plate outer markings plate in place.  use care with these.

Inserting one of the two pins, located top and bottom.  To remove the feet plate, gently pry up from the centre, and be careful to not lose the little pins.  TAke the FEET hand off first. 

Gently push down into place. 
Before you put the face and glass on, attach the FEET indicator hand.  Set the pressure as per METAR and place the hand appropriately.  

PAL 500 Power Amplifier