Odds and Ends => Tutorials => Topic started by: BaldwinK on April 27, 2017, 12:36:41 PM

Title: 995 Uses for A Dead Servo
Post by: BaldwinK on April 27, 2017, 12:36:41 PM
A year or so back I ordered a dozen MG994 servos from China, total £52 post-free.  In fact 13 arrived and I assumed this was in gratitude for my esteemed business. I made up four leg assemblies and started tests on a quadruped. Quickly it became apparent that the motors could not deliver the stated torque and a quadruped would be unable to hold up its own weight, let alone move. During testing, five of the batch failed for various reasons!

The working leg assemblies were later used for other robots but I found that a single 5A UBEC could handle no more than three 55g servos at 6V.

I have since been looking into alternatives to the usual hobby offerings. Most eBay motor gearboxes are not up to much in practice and positional feedback can be tricky to implement. I did make up a working leg assembly using just yellow motors with cermet pots for feedback – it is compact and agile but would never power a quadruped.

My five dead MG995 servos have tough metal gearboxes and pots well coupled to the main shaft. Under the same MG995 label one is quite different to the other four. It has a longer motor body, a square pot, subtly different gear train and different electronics. It was labelled ‘inconsistent stiff but no movement’. The others were labelled ‘hunting’, ‘DTs’, ‘dead zone’ etc.

Putting aside the ‘odd’ servo as probably seized mechanically I discarded the electronics and made up four new servos. These have the shorter motor body (good for fitting into a smaller space) and round pot. With the body clamped to a table and with a 10cm lever fitted to the output spline it was time for a simple torque test. A digital luggage scale reading to 50g and having a strap and hook was used. Allowing the lever to move no more than 10 degrees off setpoint showed a force of 0.75 kg.  That’s 7.5 torque - quite good for once ‘dead’ units.

Still never sure what to believe I ordered ten MG996R servos total £36 post-free; this time the stock was held in England. Under this label the innards were very familiar but not what I was expecting. These are the long motor, square pot versions so was my ‘odd’ one actually the later version?

Well it seems it was. It also now works again with external electronics using a Nano for PWM and PID and 754410 half bridges to switch 1A at 12V from a 3S lipo. The bigger motor just had to be torque tested and returned even better figures within 5 degrees off setpoint. With no current limits (operating up to PWM=255) it delivered nearly 17 before the motor pinion started slipping!

The difference in the gearboxes is in the number of teeth on each same sized gear. Counting the teeth is tedious and I may not be quite right but the long body ratio is 260:1 and the short body is 238:1. I have photographed the main parts for comparison.

The MG996R PCB is a bit different again and the pot is soldered to it directly.  I struggle with lead-free solder and the pot is hard to free.  Removing the main shaft to push out the pot whilst releasing the clips with a slim screwdriver makes it easier to unsolder in the open.

You will read about ball bearings in the blurb. I have found that the outer shaft bearing is usually a brass alloy ring, sometimes perhaps phosphor bronze. The inner shaft bearing surrounds the body plastic tube through which the pot attaches and mostly this looks like Delrin. Just one of my older units has a ball race here instead.

In subsequent testing the ‘stiff’ old unit did become troublesome. At 260:1 the motor will move the joint regardless, but moving it manually really stresses the fixings. Making use of the increased torque requires much heftier external components.

I have also found it helpful to run a 3mm tap through the threaded splined shaft to ensure the fixing bolt can hold the horn down securely. The supplied plastic disc horns seem fine but just in case I did source five metal 25T horns at £2.25 from China. A light alloy, these are tapped and come with four bolts but the attachment must be fittted to the outer flat side.

I have just received twenty MR106 ball races 6 x 10 x 2.5mm at £7.19 post free from Hong Kong. Both bearings could be changed but the most important is the inner Delrin because this was the cause of ‘stiffness’. With the new bearing replacing the Delrin ring, and no pot installed, the shaft spins freely.

So don’t throw out those ‘dead’ servos!

*Servo Parts.doc
Title: Re: 995 Uses for A Dead Servo
Post by: erco on May 01, 2017, 11:36:13 AM
Excellent post & ideas. Chinese servos are so cheap now that it pays to stockpile them. Quality is very spotty though, so you do need to test each one. MG995's have gotten rarer since this article came out:  so I think many sellers just changed the sticker to MG996! Even so, for $4 they are worth a shot. The bad ones I just gut and use as metal gearmotors, the cheapest you'll find.