ITT (ASCO) Hydramotor NH92 4000# Hydraulic Actuator

[DieselGuy]

We have been experience early life failures of a NH92 Hydramotor in a valve actuator application. The mounting is vertical and in-line to within a few thousands of an inch. A split coupling is used to assure that no side loading is introduced between the actuator and valve stem. The Hydramotor is rated at 4000# and therefore far exceeds the operating force needed for the temperature control valve that it operates. The hydramotor stroke is capable of a 4 inch stroke, although it is calibrated to operate at 2 inches. The actuator strokes at approximately 7 seconds per inch of travel against the springs and 2.5 seconds per inch with the springs. The actuator is a modulating type actuator with minimal cycling noted. The fail safe position is with the stem fully retracted by the spring pack.

We have recently experienced severe galling of the guide bushing in this application which has been attributed to side loading of the bushing. The guide bushing is a brass alloy with a chrome plated main shaft. The lower guide bushing is not lubricated. The actuator is mounted vertically and is in a 65 Deg F, clean dry environment. The bushing design has remained unchanged through its design life however the vendor indicates that the shaft has had some changes through the years. Side loading may be induced by the spring pack and machining tolerances.

Looking for any experience with these actuators that may assist in improved performance.

 

[MikeHalloran]

The term 'split coupling' brings to mind a cylindrical coupling that is split on a plane through its axis, and is held together with pinch bolts. When the bolts are torqued, the coupling is completely rigid, and will transmit side loads, e.g. from machining eccentricity, unabated.

Did you mean something else by 'split'?



Mike Halloran
Pembroke Pines, FL, USA

 

[desertfox]

Hi DieselGuy

Anychance of a drawing,picture or sketch?


desertfox

 

[DieselGuy]

The split coupling is as you noted, however is designed to allow .030" clearance and therefore should not translate sideload from the driven component unless extremely out of alignment. The application in this case was aligned within a few thousands.

Thanks for your inquiries. A few photos have been uploaded

 

[desertfox]

hi DieselGuy

Where are the photo's uploaded?

desertfox

 

[DieselGuy]

I sent them per step 3 attachment but have not been able to view them. I assume they would attach to the thread.

 

[desertfox]

Hi DieselGuy

Did attach the file before uploading it, if you didn't use the browse button and select your file before uploading, that would explain why there is nothing attached.
Have another go.

Regards

desertfox

 

[desertfox]

Hi DieselGuy

I think I managed to find a section drawing of the Actuator your taking about.
The split coupling is supplied along NH92 Hydramotor as a complete unit according to this catalogue.
On page 67 there is a cross section drawing and I assume the bushing your referring to is the one thats nearest to the split coupling above the arrow which says extension rod.
A say bushing when I zoom in its like a sleeve in the end of the tapered body.
What I cannot find is any info on alignment tolerance, but as you point out unless the compression spring is perfectly centred to the extension rod then that will certainly generate side load and although the spring appears to be guided any machining tolerances on the parts or spring squareness will certainly contribute to the problem.
I would suggest you talk to the supplier or manufacture of the device.
For the benefit of others I'll upload the pdf for reference.

desertfox

 

[hydtools]

Has the extension rod been replaced? How does the extension rod attach to the actuator rod? Is the extension rod precisely aligned with the actuator rod? If these are misaligned there will be a side load of the extension rod against the lower bushing/bearing. If the extension rod is bent it will side load the lower bushing/bearing.

I would expect that the plate the spring bears against has some radial float/clearance on the extension rod. The spring should have little chance to side load the bushing/bearing.

Ted

 

[hydtools]

Here is an interesting publication.

http://mydocs.epri.com/docs/public/TR-112181.pdf

It reports scoring in older units using stainless steel instead of the later chrome-plated rods.

Ted

 

[DieselGuy]

We have replaced all components through the years. The shaft is center ground and chrome plated. All bushings seals and shaft materials were replaced approx 1 year ago. We have our lab performing material property testing and hardness checks.

We are seeing the potential for misalignment between the power head and the yoke assembly. It is possible to misalign components such that the shaft will not install through each bushing. This seems to be unlikely in that it would be corrected during assembly.