uplock actuator

[Burt]

Just finishing my annual inspection on my 337A. During the gear swings, at the end of the gear up cycle, we could hear a loud squeak or squeal when the uplock actuator engaged. The sound seems to be coming from the actuator itself and not the associated linkages which incidentally were all lubed. My A&P/AI has a lot of 337 experience both working on them and flying them (he has owned two of them himself). He has never run into this issue. The actuator is not leaking and operates normally except for the sound. When the uplock actuator releases the uplocks on the gear down cycle, it does not make the sound. It only does it when it engages the uplocks at the end of the "up cycle". We then tried retracting the gear with the emergency hand pump and the actuator did not make the sound when it engaged. It only does it when using the mule to cycle the gear. The mule we are using was made using the electric/hydraulic pump from a Cardinal RG. I have owned the aircraft for three years so this is the third annual we have done using this same mule for the gear swings so the mule pressure would not seem to be the issue.
Hoping someone out there has run into this issue or at least has some ideas concerning it.
Burt

[RandyArizona]

Hi Burt,
Best guess: Your hoses or fittings have a crack / leak / loose. The "squeel"
You hear is air coming out, or being sucked in. (The mule... does it have any way to check if the fluid is frothy, ??)
It only "squeels" when under pressure, so the manual pumping is not enough pressure to "suck the air".
Check fittings, if hoses are older than about 8 years old, replace.
(Also check for wetness around the fittings, under hose sheath. DO NOT STICK YOUR HAND UP THERE WHEN UNDER PRESSURE!! You could lose a finger, or two.)
Good luck.

[RandyArizona]

Just so you know where I'm coming from....


Squeal from Hydraulic Actuator
A squealing noise from a hydraulic actuator is typically caused by high-pressure hydraulic fluid passing through a restriction or a malfunctioning component. One common cause is the hydraulic system's pressure relief valve operating, which can occur when the actuator reaches its maximum travel or is under excessive load, forcing fluid to bypass the valve at high velocity.
This is often heard as a high-pitched squeal or whine, particularly when lifting heavy loads or when the control lever is near its uppermost position.
In some older tractors, this can be a normal characteristic when the lift control is fully raised, entering a "regenerate" mode where the pump continuously releases pressure through the relief valve.

Another potential cause is aeration, where air enters the hydraulic fluid, often through leaky couplings, loose fittings, or low fluid levels, leading to a whining or knocking noise that can be erratic in nature.
Cavitation, which occurs when fluid pressure drops below its vapor pressure, can also produce a high-pitched whining or screeching sound due to the collapse of vapor bubbles, causing erosion and damage to internal components.
This is frequently linked to blocked filters, restricted intake lines, or excessive pump speed.

Internal wear or damage within the actuator itself, such as a scored cylinder bore or damaged piston packings, can allow high-pressure fluid to leak through small gaps at high velocity, resulting in a squealing sound.
Similarly, a cylinder with a bent rod or worn seals may bind or slip, generating noise during operation.
In some cases, a stuck or improperly adjusted relief valve can cause a constant hissing or whistling sound, which may be mistaken for a squeal.

Other mechanical issues, such as a damaged or worn pump, faulty directional valve spool, or even a failing clutch throw-out bearing in some tractors, can also contribute to unusual noises that may be perceived as coming from the hydraulic system.
It is important to investigate the source promptly, as prolonged operation under these conditions can lead to accelerated wear, component failure, or system damage.

[Burt]

>>>I think for starters I will just replace the hoses and overhaul the actuator and see what that does.<<<

Great idea.

[RandyArizona]

Originally Posted by Burt

>>>I think for starters I will just replace the hoses and overhaul the actuator and see what that does.<<<

Great idea.

Well, not sure how that happened.. but I "edited" your post... either way, hoses and actuator ovhl is a great start.