Mechanical Null Bias


Thread Starter


In all moog servo we find small nut and Allen screw. my question: what is the function of this part?

if it is not adjusted or need to readjust, what will be the symptoms? I already saw some videos on YouTube. and I still not sure this procedure done with electrical connection or with out

It is only possible to adjust the null bias spring tension with a proper test apparatus (oil, at the proper pressure and temperature, with a very accurate method for measuring the coils currents AND the oil flow-rates).

In my experience, virtually every servo that someone has tried adjusting the number bias spring tension without the proper equipment has had to be discarded.

Servo-valves fail primarily for one simple reason: Poor oil quality. High temperatures can also reduce servo life expectancy, but if the oil isn't clean and maintained eventually the servo is going to fail. And the replacements don't seem to last very long, either.

Many servos have very thin "pencil" filters, which can be cleaned and replaced--BUT, they are really just rock-catchers. The main hydraulic filters must be high w quality and changed as soon as the differential starts to increase to about 1bar. Preferably at 0.6-07 bar dp. And, filters also break down over time, so even if the unit doesn't run very often the filters should be replaced as recommended by the manufacturer.

A few people have reported being successfully able to adjust the null bias spring tension--but, the spring tension should not change over time so if the tension "requires" adjustment the question should be, "What has changed to cause the need to adjust?" If the spring is worn or broken, is adjustment going to fix that? If the spring tension has decrease, that's most likely the result of excessively high temperatures (oil; location) and adjustment, even with the proper equipment isn't going to last long. So, those reports are dubious at best.

Finally, many people mistakenly believe that if the servo current from one processor is not the same as the other(s) that adjusting the spring tension can fix that problem. There is only one spring, and the sum of the currents acts to overcome spring tension--so attempting to adjust the single spring is going to have an effect on all the servo currents, not just one.

What is it EXACTLY that you believe adjusting the null bias spring tension will do, or why do you believe it might be necessary? Maybe if you described the issue you're trying to solve we can suggest how to troubleshoot and resolve the issue.

Adjusting the null bias spring tension is really best left to companies with the proper equipment and experience to test and repair--if necessary. If you're buying refurbished servos that have what you believe to be improper tension you should go to the supplier/refurbished with your concerns and actionable data to support your position. My experience with most servos refurbished by firms other than the original equipment manufacturer has been poor, at best. At one site we replaced six servos, using refurbished servos, on one Gas control valve before one of them was found to be serviceable. Those cost nearly $1000.00 USD, each. And four people worked on the changeouts over two days, with a LOT of lost steam- and electrical production and revenue. That site (which has eight GTs) threw out all the remaining refurbished servos and now only buys new servos. They rarely fail out of the box, and the manufacturer accepts returns if they do. Sometimes, the true cost of refurbished is more than just the purchase price....

But, if servo failure is or is becoming an issue, best to examine the oil quality. If it's nearing (or past) time to replace the oil seriously consider flushing the hydraulic system as part of the replacement.

Have you read this thread?

Mohamed Oraby

Dear CSA

Thank you for your help. you are right. Actually the problem always related to oil quality. but after I replace the servo's filter and return it back. while I perform calibration, I found the GCV1 feedback oscillated with very fast rate. then I replace the servo with new one, perform calibration again then every thing is perfect. so I think adjusting mechanical null bias may help me to save the defected servo. I watch some videos in youtube, but i still confused.

I find a great deal of information on YouTube, but as with most free information available on the World Wide Web they are prepared by novices and can be sorely lacking in details. Many seem to be filmed in response to discovering some feature or nob that was previously not noticed by the poster and therefore not noticed by potential viewers--so they are really just telling the world, "Look what I discovered!" even though they have little in the way of details about functionality and/or operating principles.

Electro-hydraulic servo-valves as used on GE-design heavy duty gas turbines enjoy mythical status. The (false) belief that they are in any way affected by a "calibration" is one of many mythical attributes. (The <b>ONLY</b> thing calibration does is scale LVDT feedback--<b><i>nothing</i></b> else.) And, most people do not verify servo current polarity--which affects stability MUCH more than LVDT calibration--for SIMPLEX, DUAL Redundant and TMR applications. And, there really is NO method to calculating null bias current--in spite of what the GE Control Specifications say. And, finally, most people don't physically measure actual position versus reference when "calibrating" and just assume the Mark* knows the actual physical position when performing an AutoCalibration (which it can't).

The null bias spring only serves to push the internal spool piece to the position that ports hydraulic oil to the hydraulic actuator to shut off the flow of air or fuel to the turbine. It has no affect on stability. Adjusting the null bias spring tension only serves to require more servo current (positive or negative), and unless one then changes the null bias current value--again, there is NO way to correlate a change in adjusting screw position to some amount of null bias current--the device will have an error between the reference and the feedback back. Which calibration (Auto or manual) cannot and does not change unless one manually creates a false calibration.

The null bias spring tension can only be accurately set with proper test equipment and procedures--something few sites have. There was a thread a long time back about a site with more dollars than sense that was or had purchased a servo-valve test set-up to use to try to self-refurbish servo-valves. We never heard back from that poster about how successful that was.... even though we asked for feedback. (One would think if the effort was successful the poster would have been anxious to tell as many people as possible--perhaps even posting a video to YouTube!)

How can changing null bias spring tension clear or clean the internal passages of the servo-valve to restore proper operation, if the cause is poor oil quality? If you can explain how changing the null bias spring tension will clear or clean the internal passages of the servo-valve that would be very helpful. And, if soon after replacing a servo-valve the "new" servo-valve starts misbehaving then wouldn't the logical presumption be that something other than the servo-valve is to blame for repeated and frequent failures of servo-valves? Putting good equipment in a bad environment is going to cause failures or problems all the time. Fix the environment, and you'll fix the problem. How is adjusting the null bias spring tension going to fix the oil quality issue?

By the way, do you clean and/or replace the "pencil" filters in servo-valves? And, if you do, do you do so in a clean environment, after cleaning the outside of the servo-valve before removing the pencil filter? Cleaning and/or replacing the pencil filter in a proper environment after cleaning the outside of the servo-valve would more to restore proper operation (at least for a while) than haphazardly adjusting the null bias spring tension.

Servo-valves cost approximately a couple of thousand US dollars. Replacing two or three with new, or even refurbished ones, including the lost production before and during the replacement AND the cost of labor during the replacement, would surely be nearly as expensive as changing the lube/hydraulic oil and performing a thorough oil tank cleaning and hydraulic oil system flush. My spreadsheet says avoiding future servo-valve replacements is much more cost-effective than trying to self-refurbish or replacing servo-valves.

And as part of the oil replacement there should be a plan developed to test and maintain oil quality to extend the life of the oil as much as possible--and reduce future servo-valve issues.

But, if you can explain how adjusting the null bias spring tension will cleanse internal servo-valve passages and restore proper operation and how to then reset or change the null bias current value to achieve the recommended range of null bias current, you would then be able to produce your own YouTube video, and help The power generation world solve a big problem. (The servo-valve manufacturers won't be happy, though.)

We are all waiting to hear how adjusting null bias spring tension can restore proper servo-valve operation, but more importantly how to adjust the null bias current value.

Have you read this thread and looked at the picture in the link? Does it help to explain how adjusting the null bias spring tension can restore proper servo-valve operation?