At about half way through the last challenge event during a mediocre pull there was a loud bang at the front and the winch casing on the motor side of the H12 MM split into two pieces
. The crack or split in the casing was right across the bolt holes for the motor mounting and further investigation showed this to be due to a weld failure on the end plate of the drum (a known common fault with the earlier MM’s). Witness marks show that the weld had been broken for some time and the plate was running against the end on the two motor mounting bolts (7/16 UNC SS). The failure mode was obvious as the welding was of very poor quality with practically no penetration on the end cheek. The broken casing casting is cheap and readily available from MM.With regard to the drum it was decided to repair and improve rather than replace and the purpose of this post is to demonstrate how to achieve a reasonable standard with the use of basic home workshop tools.
One of the big issues with the MM is the relatively small size of the drum. Although adequate for 100ft of 10mm, bunching at the tie bars can present a serious issue if not carefully monitored. With 12mm string the useful length that can be fitted is only 73ft (22m). The type R winches overcome this issue by using a larger (deeper) drum and Alfred Murray sell a kit to achieve this with the H12 drum. Although I have a kit to hand the method is to plug weld split oversize end cheeks that are laser cut from 5mm plate, however, accurate alignment is difficult and the size increase could be larger, so I decided that it would be easier to construct new and fit end plates whilst the original end plate was removed.
It is widely reported that the drum is constructed from EN34 steel which is reasonably hard, however, the original end plate seems to be much harder than that as it refused to be centre punched or drilled (even with a cobalt drill bit)…………. turning was difficult but achievable with care. The thickness of the plate is 9/32 (about 7mm).
A quick hunt around the workshop produced a couple of square pieces of 6mm plate that could be pressed into service. These were quickly centred and drilled. Also four of the plug weld 8mm holes were marked and drilled. Interestingly a number of folk I know own lathes, but not many seem to know how to use them to produce anything useful
! The lathe in the workshop is a small Myford ML4 model engineering lathe that was purchased new by my father in 1945. I have owned it for the past 15 years and this lathe over the past 65 years has produced countless model 2 and 4 stroke engines. I run this lathe from a washing machine motor with a home built closed loop speed controller.The two plates were bolted to a face plate (using 10mm spacers) via the predrilled 8mm plug weld holes and then centred on the face plate by using a revolving centre in the in tailstock. Once satisfied that the plates are correctly centred then the next job is to turn the outside diameter of the plates. I decided that 190mm (7.5in) was about the largest diameter that is practicable. Care should be exercised with taking intermittent cuts (square / hex to round) as this produces high shock loads on the turning tools. Once the plates are at the correct diameter then the next stage is to bore the plates oversize to the drum size. The measured drum size was 2.497in so in order to provide a strong weld with good penetration the plate centre holes need to be about a ¼ in larger then to drum to provide for a 1/8in weld gap. This is simple job for the boring bar to machine the centre hole out to 2.750in (+ - 1thou). Although this job does not require much in the way of accuracy it is good practise to mount any job so that all the turning processes (or as many as possible) can be achieved without dismounting the work piece.
At this point it seemed like a good idea to clean up the old weld from the drum. Again the lathe was pressed in service………
The remounting of the end plate onto the drum poses its own engineering problems. The end of the drum is machined to slide into a PTFE bearing and the end plate rotates within the side housing and is sealed with a large O ring. Therefore the plate must be mounted at a perfect right angle to the drum, at exactly the correct depth on the drum, and must also be perfectly concentric. It was considered to build the new end plate (motor end) from on solid piece, however this was discounted as (a) I didn’t have any 14mm stock that was large enough, and (
the material would have needed to be hardened.Fortunately the solution is simple……………The bearing portion of the drum is machined to 2.450 thus leaving a 50 thou step at the exact mounting point of the end plate.
All that is required is a simple jig to ensure that the plate sits at a perfect right angle to the drum, especially during the welding process. This is achieved by turning up a jig with a centre hole on 2.455 (sliding fit) with a flange for the end plate to register against.
Again a hunt in the workshop produced a cast steel 2.5 to 1.5 BSP adaptor. This was mounted in the lathe and turned to produce a ‘machining’ face. This face was then mounted in the chuck and the centre bored the correct dimension. Then the flange was faced…..so now we have a flange that is at a perfect right angle to the bore.
Now we have the desired jig ……………
Ok so we now have two larger end cheeks and a method of mounting the original end plate. Unfortunately, because the new larger end cheeks are bored oversize by ¼ in we have the ability to mount them incorrect with an eccentricity of up to 1/8in (3mm). Therefore another jig is needed to ensure concentricity and to make life easier. A further hunt around the work shop produced an old block of aluminium that had been used as a jig in another life. This was centred into a 4 jaw chuck and then bored to correct diameter of the drum (2.503 for a sliding fit). I then transferred it to a 3 jaw chuck to make life a bit easier when turning the face. A quick check with the dial gauge showed a mounting accuracy to within a thou which was more than accurate enough for this application. Once mounted the excess metal was quickly removed from the periphery. Next a register is turned into the face. The diameter of the register needs to be a few thou less than the hole on the end plate………… like so……
Then we cut the jig into two……………
The jig fits like so………….
The rest is all straightforward. Another four holes are carefully marked and drilled to 8mm for plug welding. The plates are then set with the jigs and plugged together. Then the finally the drum is welded to the plates. To ensure good penetration with the limited current available (160A) the wire speed was lowered to allow for slower welding, thus producing much more heat. Its still a little faster than gas or stick, but much slower than you would normally expect to weld with mig. Also the work was heated prior to the welding process. I left some grease on the inside of the drum knowing that if it caught fire then the required weld penetration had been achieved…………I can assure you that burning Fuchs aqua N2 grease stinks and makes your eyes water !
Almost finished……………. Just need finishing and painting……….
It is well understood that longer is preferable to deeper and TBH the MM lends itself well to a longer drum modification, however, all long drum winches run into issues with winch mounts and hawse fairleads. A deeper drum is OK provided that you understand the limitations. Calculations show that even when on the 7th layer the winch will still pull 2T ish ((motor torque (in/lbs) x gearing) / drum radius (ins)) . The new depth allows for safe capacity of 187ft of 10mm or 134ft of 12mm. I will limit the capacity to 100ft of 12mm which will go a long way to alleviate the original issues with bunching.
Another simple job that takes a week of evenings to achieve !
Ian
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.............oh revenge is so sweet 
