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Scammell Constructor Gallery.


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Thanks for a nice clear explanation. Just one more Scammell gearbox question, how many usefully interchangable parts do the Constructor and Explorer gearbox have in common ? I mean, if one had the job of reconditioning a Constructor gearbox, would an Explorer gearbox in good condition yield up many useful parts ? Cheers. (this maybe a simple question with a more complicated answer but I thought ask it anyway)

 

As far as I know, (Mike may well know different) everything except the output shaft (mainshaft in Scammell parlance) is interchangeable, hence I re-used the one from my old box. If the old one had been completely unserviceable it would have been a real problem for me, but as it was for my limited use, it was OK.

 

I don't know about different ratios etc.

 

The g/box casings are not interchangeable as the transfer box fixings differ.

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As far as I know, (Mike may well know different) everything except the output shaft (mainshaft in Scammell parlance) is interchangeable, hence I re-used the one from my old box. If the old one had been completely unserviceable it would have been a real problem for me, but as it was for my limited use, it was OK.

 

I don't know about different ratios etc.

 

The g/box casings are not interchangeable as the transfer box fixings differ.

 

 

Spot on Bernard and you are living proof that it can be done. There were some minor mods over the years with additional oil drillings that have to be lined up and remember that the casings are machined as a pair and must always stay together.

 

Mike

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Thanks very much. I'd heard various stories over the years and this seemed like a good opportunity to find out from people who have obviously had first hand experience. Cheers.

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As I understand the problem with the Contructor gearbox, is due to it having two Cardan shafts to the two rear axles.

The Explorer had only one shaft running from the back of the box, because it only had a single rear axle, with a single diff.

 

Any imbalance in tyre size between the front and rear wheel, on the walking beam was taken up by the massive gears in the walking beam, that link the two wheels together on one side, at the rear.

 

This gives no real problems.

 

The Constructor had two distinct axles and two Cardan shafts to drive them. If there was an inbalance in the tyre sizes on the first and second rear bogey axles, these two Cardan shafts tried to turn at different speeds, but they were joined at the transfer gearbox end by much smaller gears.

 

I believe a lot of the gearbox troubles starts from not carefully matching tyre sizes because the Constructor is one of the few vehicles where the imbalance has to be taken up in the transfer gearbox gearing, between these two rear output shafts.

 

Add to this hardy spicer joints only maintain a constant rpm, between one end and the other of a propshaft, when the joints lie in the same plane, with the spiders lined up.

 

The rear propshaft of the constructor goes up and over the leading bogey axle and uses three Hardy Spicers, non of which lie in the same plane, when they should only be used in pairs.

 

The problem with the Scammell was not the workshop floor staff, the problems came down from the drawing office!

Edited by antarmike
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Mike, there was an article in the Scammell Owners Club magazine by the chap who collected one of two new military constructors, one was OK but the other had bad vibration problems, it was returned to the works but came out again just as bad! I'll the magazine out and refresh my memory, I think maybe the reg nos. are mentioned.

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Mike, there was an article in the Scammell Owners Club magazine by the chap who collected one of two new military constructors, one was OK but the other had bad vibration problems, it was returned to the works but came out again just as bad! I'll the magazine out and refresh my memory, I think maybe the reg nos. are mentioned.

 

I think you will find it was Stan Wass and his colleague collecting their brand new MOS Constructors which were to be used for transporting tanks on the 50 ton drawbar trailers.

 

Mike

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I think you will find it was Stan Wass and his colleague collecting their brand new MOS Constructors which were to be used for transporting tanks on the 50 ton drawbar trailers.

 

Mike

 

Right again Mike, he drove NGY 593 which was the better one. The other one NGY 592 only made it from Watford to Newark because the center prop bearing went, this kept on happening.

Vibration was always a fault with both of them around 28-30 mph, over 30 all was well until the speed dropped to 30 again, sound like propshafts to me!

Going by the reg nos it would have been 1953 ish

 

And the better one still lives

 

dorsetconstructor.jpg

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As I understand the problem with the Contructor gearbox, is due to it having two Cardan shafts to the two rear axles.

The Explorer had only one shaft running from the back of the box, because it only had a single rear axle, with a single diff.

 

>Yes, but any wind up on the Constructor is only felt by the transfer gears, not the main gearbox. The Constructor transfer box is a robust component with huge gears which due to the diff ratios only take about a tenth of the torque seen in walking beams of Explorer. The only problem I heard of with the transfer box was overheating due to the small quantity of oil it contained - just 4.5 pints on later models with the extended filler neck.

 

Any imbalance in tyre size between the front and rear wheel, on the walking beam was taken up by the massive gears in the walking beam, that link the two wheels together on one side, at the rear.

 

This gives no real problems.

 

The Constructor had two distinct axles and two Cardan shafts to drive them. If there was an inbalance in the tyre sizes on the first and second rear bogey axles, these two Cardan shafts tried to turn at different speeds, but they were joined at the transfer gearbox end by much smaller gears.

 

>Any twin axle drive without an inter axle 3rd diff is going to suffer from wind up, even if it were possible to select identical tyres. Many road going 6 and 8 wheelers from the 50s and 60s ran twin drive without a third diff, but they paid the price in accelerated tyre and unnecessary gear wear in the final drives. Militant 1 (and I believe 2) had no third diff as AEC insisted that cross country performance was superior without. It also did away with a component which could be fragile and easily damaged if not locked up prior to driving off road, as with the Crusader tractors.

 

I believe a lot of the gearbox troubles starts from not carefully matching tyre sizes because the Constructor is one of the few vehicles where the imbalance has to be taken up in the transfer gearbox gearing, between these two rear output shafts.

 

>As above, the gearbox is not subject to any stress from axle wind up.

 

Add to this hardy spicer joints only maintain a constant rpm, between one end and the other of a propshaft, when the joints lie in the same plane, with the spiders lined up.

 

>Absolutely correct, but remembering that the actual propshaft speed varies in relation to the joint angle, speeding up and slowing down either side of the input shaft speed. Explorer is a great example of pushing joints to the limit, and I believe that the shaft manufacturers were most unhappy about the angles Scammell were using.

 

The rear propshaft of the constructor goes up and over the leading bogey axle and uses three Hardy Spicers, non of which lie in the same plane, when they should only be used in pairs.

 

>The rearmost prop on a Constructor is a technical disaster, but there was no alternative while Scammell used the traditional axles which did not lend themselves to a through drive system. At low speeds it was acceptable, but the vibrations at higher speeds would knock the centre bearing out. The field service bulletin from Scammell when faced with a severely vibrating vehicle was to remove all 4 halfshafts and run the rear axles at full speed, and then index the rear prop centre flanges one hole at a time until a position was found with the least vibration.:rofl:

 

The problem with the Scammell was not the workshop floor staff, the problems came down from the drawing office!

 

>Unfortunately, yes (dons tin hat to deflect the missiles which are about to be thrown in my direction)

 

Mike

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Thanks 6X6, the problem looks pretty obvious looking at those pics, and there's no give anywhere, it was, in theory, doomed from the start :-(

 

Have a look at this page for chapter and verse on the subject. http://www.machineservice.com/technical.htm

 

scroll down to Driveshaft Vibrations

Edited by gritineye
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I hear what you say Radiomike, but when I say the gearbox trouble is largely due to the two propshafts, I am talking about the Main gearbox, transfer gearbox as one unit, and a lot of the takeouts were due to transfer box problems, not maingearbox problems, And most of the vibration is due to bearing failure, not stripped gears or broken shafts.

 

If the bearings carrying the input shaft of the transfer gearbox fails, this also causes problems woth the output shaft in the main gearbox...affecting the depth of mesh of it's gears also...

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sc00227afd.jpg

 

Sorry if I seem like a dog with a bone, but what I can't quite understand is that if the Constructor's transmission is so riddled with faults, how come three Constructors I have had personal contact with, PSU 439, YSV 865 and UVS 149 (and I'm sure there must be others) all drove without any of the faults being attributed to Constructors here ? All three vehicles had both props in place driving both rear axles, all had significant mileage on the clock, all three are now pushing 60 years of age, all three could be/can be driven in every gear, right through the rev range, with NO SIGNIFICANT VIBRATION.

 

Well, you might wonder if these three Constructors had been modified in some way. They had not. They are, in the respect of their transmission, completely original. I repeat, substantial mileage, both props fitted and in all gears NO SIGNIFICANT VIBRATION.

 

I certainly don't deny the Constructor transmission has it's faults and weaknesses and that most of these faults have been correctly identified in previous posts. But most 'faults' can be anticipated and remedied with routine maintenance. Take the problem of output flange bearings breaking up. How long does it take to check for early signs of play ? The signs that one of these bearings needs renewing will be present long before the bearing breaks up. So, bearing break up, design fault or poor maintenance ?

 

 

God knows how many Constructor gearboxes have been destroyed owners ignoring the warning drips of oil that indicate the seal between the boxes needs replacing and oil is now migrating into one box leaving the other box bone dry. I mean, Scammells can't talk, oil leaking from the gearbox is it's way of telling you it's not very well. Worn out seal on gearbox connecting shaft, poor design or just wear and tear ?

 

 

During a 20 minute telephone conversation I once had with Stan Wass I made a point of asking him what he thought about the Constructor gearbox and he only had positive things to say about it. As did another Constructor driver Bert Paul. Maybe I should have asked Stan what he thought about the twin prop shaft arrangement. Most Post-War, low volume, special purpose British MV's are, in my humble opinion, to some degree lemons and it's hardly surprising that there were teething troubles. Maybe that's what Stan's

mates problems could be attributed to. Just a few years ago the TNT depot where I work received a batch of 20 new model MAN's. Within less than two weeks 15 units were down at the MAN dealers in Avonmouth with faults that included a seized engine. Now they're on the road 24/7 and I can't remember when any of them last broke down. Teething problems occur to even the best of manufacturers.

 

Regarding mismatching tires on the rear axles I not even going there. Anyone who can't grasp the importance of doing this shouldn't be allowed anywhere near one of these vehicles.

 

For what it's worth, my overall feeling about the Constructor is that Watford did a reasonable job of supplying a highly specialized, very capable vehicle that was a product of it's time. Lets not forget that with all it's faults and failings, a single Constructor was able

to handle a load well in excess of 100 tons and did so for many different operators overs a period of years WITHOUT gearboxes, or other transmission components, giving any undue trouble. A number of these heavy haulage Constructors, now 50 plus years old and in preservation, can be driven today still with their original gearboxes. True or false ?

 

Just to remind you, PSU 439, YSV 865 and UVS 149 all with both props fitted, substantial mileage on the clock, no modifications, all now approaching 60 years of age, through all the gears over the whole rev range NO SIGNIFICANT VIBRATION. In the light of some of the previous posts on this thread, if you care to, please explain.

 

I'm on the wall of death for the next few days so if someone does reply to this post the reason I haven't replied is that I'm off line. (and not because I'm sulking)

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Well, 6X6, I'll be glad to stand alongside you (well, just to the rear ever-so-slightly in case there's a load of incoming....:-D) in singing the praises of Scammell.

 

On the other hand, that rear axle drive is iffy.

 

I do not agree that 3 U/Js has much to do with it - the first one behind transfer box drives pretty much level with axis of box (is the slight angle towards the centre line of chassis compensated by angle of rearmost prop? I'll have a look some time, but they weren't daft in the drawing office).

 

But note the rear prop has U/J angles equalised by tilting the rear diff upwards, i.e. away from axis of gearbox (unlike other vehicles as in Landrover, Bedord etc where the axis of diff is the same as gearbox). This in theory is fine as each U/J still takes an equal angle and constant velocity ensues, but I believe that propshaft/joint balancing then becomes critical.

 

I do not know of any other vehicle where the U/J angles are equalised in this way - PTO shafts on agricultural tractor / trailed implements go like this when turning, as do the drive shafts to trailer on the Multidrive set-up. But these cases are only when turning, normal use is straight drive.

 

I reckon that any dynamic forces caused for example by unbalanced propshaft, would to some extent be compensated between U/Js at each end in a normal set-up (eg Bedford MK or Landrover)(i.e. when prop is pulling one U/J it will be pushing the other). However in this case the prop would be trying to "escape" in the same direction at each end, so U/Js would have their work cut out to contain the substantial forces.

 

Scammell's ploy of 'rotating the whole propshaft assy one bolt at a time until an acceptable state was reached' only serves to alter one set of out-of-balance forces in relation to another (the propshaft assy to second axle). If the drive to second axle was perfectly balanced then altering the relative angle of U/Js on rearmost drive would have no effect at all.

 

The reason I'm interested in this is that PGK888 came home with a thrown rear prop. It had even destroyed the substantial frame cross member carrying the centre bearing :shake:. Fortunately the sale included a full set of replacment parts (including cross member) so I have no excuse not to get on with the job. But I am anxious to understand WHY it happens, and will be getting all rotating parts fully balanced before installation.

 

Having said that, I cannot believe Scammell would not have done the same to try to resolve the problem:confused:

 

I'm in total agreement with you on the subject of gearboxes - but nothing is perfect and possibly the achilles heel was the lubrication system maybe?? so many oilways to get blocked, and as you say, it does need oil as well to work! But what a wonderful bit of engineering, says someting for the boys from Watford.

 

Also, CONSTRUCTOR was a purely commercial design, not a post war MV design complexity :cool2:

 

Mis-matched tyres? Well yes, be sensible. But keep it in context too - if you put a toally mis-matched set of tyres on a 6 wheel tipper and drove around all day empty I think you'd just wear some rubber out, but stick 16 tonnes on frieght on board and something might just 'give' :-D On the other hand, I would not like to run an old GMC 6x6 empty without a matched set, those props and U/Js are not over big!

 

My dad once put a high speed diff in the front of a Matador dumper (10T cap) by mistake, and the driver brought it round after a few loads complaining that the rear kept jumping up when empty and the front bucked when loaded. Just think of the wind-up on that! (I watched them driving it around the yard to check :shake:).

Edited by N.O.S.
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