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P O L Y T E C H . N U

Kearfott 425408-1A-B vertical gyroscope

I'm very interested in high quality technical solutions and avionics are 'the place to be'. Avionic devices are usually well engineered and well built. There's a lot to learn form and high precision electromechanics is just like art for me. Gyroscopes are therefore very interesting devices. They're high precision electromechanical devices that are very interesting. Luckily eBay is a great place to find these items.


Gyroscope types and use
There are several types of (mechanical) gyroscopes. The first group uses a gyroscope where the force of the movement of the gimbal is used to indicate 'something' like the turn rate. Another group of gyroscopes can 'move freely' like a directional gyroscope and a vertical gyroscope. A directional gyroscope is used to determine the rotation around the vertical axis; the compass direction. Directional gyroscopes are less complex than a vertical gyroscope and therefore less expensive. The 'nicest' type of gyroscope for me is a vertical gyroscope. These gyroscopes are used to determine the pitch and roll. Even after decades, these vertical gyroscopes tend to be rather expensive.

The Kearfott 425408-1A-B vertical gyro(scope)
I bought already some 'cheaper' gyroscopes like a turn and slip instrument with a gyroscope. But after looking for a while I found 'a real' gyroscope: a Kearfott 425408-1A-B vertical gyroscope. This us a US made gyro dated 31 october 1962. That's almost 60 years old when bought by me!


The gyroscope device when received.


The typeplate of the gyroscope.

Gyroscopes are delicate precision instruments and dan easily be damaged by shocks. Movement isn't that big of a problem. But large shocks can damage the device. Usually a gyroscope instrument is marked with the note: "Handle like eggs". And I think that's a rather good description of how to handle gyroscopes. The Kearfott gyroscope is also marked with a suitable warning: "Caution, do not move or handle gyro while rotor is still running". So, just handle these devices like eggs...


Caution label to emphase the delicacy of the instrument.

The housing is rather interesting. It looks like the housing is hermetically sealed. It looks like the housing is soldered to seal the device. There's a small tube at the bottom of the device that indicates that the housing is filled with some gas; probably nitrogen. By filling the housing with a gas like nitrogen, there's no (moist) air that can lead to oxidation and therefore corrosion. Corrosion could result in bad electrical contacts and therefore result in functional failure. Vacuum could also work, but since there are materials that can vaporize gasses the vacuum van easily contaminated and vacuum is therefore not likely. Nitrogen is rather cheap and therefore a logical choice. Air planes are stressed by bad weather conditions like low/high temperatures and for example high levels of humidity that easily damages equipment. So a local controlled environment like this 'nitrogen bubble' is good for preserving the parts in the housing. At the top of the housing is a glass window installed. This is a very nice feature for visual inspection. I expect that the installation of this window was rather expensive, but is a very nice quality detail!


The seam in the housing seems soldered to create an airtight seal.


This is the view trough the inspecion window at the top of the housing.

The connections of the device are unknown (yet). I contacted the manufacturer Kearfott if they could provide the connection information or other relevant technical information. But unfortunately, due to ITAR* regulations, Kearfott is not allowed to provide any support to a civilian. The correspondence with a Kearfott employee was rather pleasant and quick, so I believe they're taking the request seriously, but due to regulations sharing this information is not allowed. The management and security team was consulted for this, so my request was taken seriously. So no hard feelings and thumbs up for the time spent by Kearfott for taking my request seriously.

* "The International Traffic in Arms Regulations (ITAR) is the United States regulation that controls the manufacture, sale, and distribution of defence and space-related articles and services as defined in the United States Munitions List (USML). Besides rocket launchers, torpedoes, and other military hardware, the list also restricts the plans, diagrams, photos, and other documentation used to build ITAR-controlled military gear. This is referred to by ITAR as 'technical data'."

This gyroscope device is almost 60 years old and to be frank, it's a very nice piece of engineering, but it's not rocket science. So reverse engineering shouldn't be too much of a hassle. It's a motor in a gimbal with position indicators, not more than that. It could be a part of a military plane, but knowing how this gyroscope works wouldn't be any risk I guess. The same principal is used in every vertical gyroscope device in each air plane... But I know how these regulations and restrictions work, so it's understandable that Kearfott is not allowed to provide information even if they like to share it. So let the reverse engineering begin!

I expect that the device uses 28VDC or 115VAC 400Hz (single or three phase) to power the gyroscope motor. I expect (hope) that selsyn synchro transmitters are used to send the gimbal position to (for example) a artificial horizon. I think/hope that a 26VAC 400Hz signal is needed to power the reference winding of the synchros. And that three wires are used for sending the position information. I wonder if the gyroscope is self levelling so I wouldn't be surprised if there's a caging mechanism. I think that by powering a solenoid the gyroscope is caged to the default position during startup. Since I have no official information (yet) and I haven't opened the housing (yet), this is still a guess. After opening the device it'll probably become clear what the pinout is and after some reverse engineering the needed signals will probably become clear. But I hope to obtain the original connection information to prevent damage during reverse engineering. ;-)
In the images below are the connector and the pinout shown. The pins are luckily clear marked on the device.


The connector at the bottom end of the device.


There is a label with the pin markings at the bottom.

Demonstration setup
I planned to remove the housing so the inside mechanics is visible. I also planned to place a glass bell jar over the gyro to prevent damage by moist and dust. Hopefully the gyroscope is pin compatible with the Smiths H6 artificial horizon. I planned to build a demonstration setup so the working of the gyroscope and artificial horizon can be demonstrated. This is a future plan...

20211117 - Last week I disassembled the gyroscope housing. The video of the event is shown below. The opening was quite simple. By heating the soldered seal with a gas torch, the solder melts and the seal is easily removed. After the opening the seem is heated again and wiped with a piece of paper to remove the solder leavings. With some wiggling the dome is lifted from the base.

My assumptions have turned out to be quite accurate, to my satisfaction. ;-) There's a motor, some synchro/resolver coils and there is indeed a solenoid for some caging or brake mechanism. The mechanism brakes some moving parts when the solenoid is not powered. The brake system is shaped that the resistance is the lowest in the default position. I expect that the solenoid is not powered (thus braked) during startup. During a gyro startup it's quite common that the gyro mechanism shakes. Due to the 'odd shaped brake bracket' the moving parts will probably end in the default position due to the lowest mechanical resistance. When the gyro is up to speed, the brake is released by applying power to the solenoid to remove the resistance. In the video I didn't see the brake mechanism, so the mechanism has some resistance as you probably can see.