Tuesday, May 29, 2012 at 12:59
Ups and Downs - Once you're towing a trailer, the gross weight the engine is pulling is substantially increased. The load on the entire drivetrain is increased dramatically.
In direct drive (4th gear or top gear in older vehicles without OD), the torque from the engine is being taken in a straight line through the transmission - or in some odd cases, if through a gearset, then the gears are evenly sized (i.e., a 40 tooth gear is driving an identical-diameter 40 tooth gear).
Thus the torque loading on both gears (and gear teeth) is even.
In the case of straight-through drive, as in 4th gear with most 5 speed OD transmissions, the drive is simply the input shaft (clutch shaft) and output shaft (mainshaft) locked together by evenly-sized splined teeth.
An overdrive gear uses an indirect gear set, whereby a larger gear is driving a smaller gear.
In the case of say, a .8 ratio overdrive, the drive gear is 20% bigger in diameter than the driven gear.
In the case where maximum engine torque is being transmitted through the transmission - and the vehicle is heavily loaded with a trailer and approaching gross maximum combination mass (GCM) - then the teeth on the smaller diameter output (driven) gear are being stressed considerably more, than the teeth on the larger (drive) gear.
As a result, with constant use of OD in this situation, you stand a good chance of gear tooth failure on the smaller diameter (driven) gear. By changing back to 4th you remove the highly-stressed OD driven gear from use, and revert to a direct drive coupling or ratio, where there is not the same extreme gear tooth loading in the transmission.
In lower gears, a smaller diameter gear drives a larger gear, and the loading is on the smaller driving gear (and its teeth).
Thus, in an extreme transmission load, the failure is going to be in the smaller gear. However, this type of failure is far less common, because the smaller gear is designed to take heavy tooth loading.
In the case of OD gears, the gear teeth loading on the driven gear increases rapidly and substantially, as high torque levels are encountered.
I can speak from personal experience here. I used to own a 1968 model International tandem drive tip truck in the early-to-mid 1970's. This truck had a 6 cyl Perkins driving through a 5 speed main transmission and a 3 speed auxiliary or "joey" transmission.
The 3 speed "joey" consisted of underdrive, direct drive and overdrive gear. International warned about avoiding using OD gear in the "joey", in combination with lower gears in the main transmission.
OD in the joey was always the last gear you changed up to, and the first gear to change down as soon as the engine started labouring.
I got a job hauling sand to backfill a mine. The job involved backing up a 10M high ramp to tip the load. It was a pretty steep ramp, and being
young and keen, I was always "speed oriented". I was on contract, of course!
I found that I could get more speed up, and climb the ramp faster, by using OD on the "joey" in combination with reverse in the main transmission.
The truck was always overloaded, because I was on mine roads, and axle loading limits didn't apply.
I did this for about 6 mths, before I heard an almighty BANG! one day, as I was reversing up the ramp, fully loaded.
The truck stopped driving. It was obvious the drivetrain failure was in the "joey" box.
I pulled the "joey" box out and upon dis-assembly, I found the majority of the gear teeth on the smaller OD (output) gear, completely stripped.
Thus the reason for the warning from International about the proper use of OD gear was driven
home to me, in the most forceful manner!
Cheers - Ron.
AnswerID:
487089
Follow Up By: Axle - Tuesday, May 29, 2012 at 20:32
Tuesday, May 29, 2012 at 20:32
Absolutley correct Ron!!,...I learnt the same way, only in the old acco tipper we had on a long run up a
hill if the ol perkins picked up revs and i was in say second or third direct i would slip, her into overdrive to gain that bit of speed instead of being patient and leave it where it was or try a higher gear in the main box, many a night crawling around in the grease and dirt removing busted gearboxes, took me enough drama to learn!..lol
Axle.
FollowupID:
762374
Follow Up By: Madfisher - Tuesday, May 29, 2012 at 21:51
Tuesday, May 29, 2012 at 21:51
Watched a you tube video of a fella driving a two sticker, bit of and art form. he hooked his arm through the wheel and changed one stick, and the other hand on the other stick.
Where the main box and Joey synro or crash?
Its interesting today most younger blokes can not even drive a synro manual.
cheers Pete
FollowupID:
762381
Follow Up By: Ron N - Tuesday, May 29, 2012 at 22:18
Tuesday, May 29, 2012 at 22:18
Madfisher - No synchro's in those days, a truck with any synchro's at all wasn't regarded as a real truck. Picking the cog spread was your next biggest job, after getting the revs right for smooth changes.
Great
old truck the old ACCO, though, I spent a lot of time in her, and she only let me down 3 times.
Once when I busted the joey, second when I stripped the worm wheel that drives the fuel injection pump, and third when I screwed off a tailshaft.
I carted up to 350 tonnes of sand a day by myself on that mine for over a year, and carted 13,000 tonnes of ore from Higginsville to the
Norseman State Battery between 1972 and 1981.
Used to have to get down to 3rd underdrive in the boxes to climb those big hills just North of
Norseman - and no doubt many a fully loaded Interstater abused me soundly for making him brake or have to follow me up those hills!
I never carried a radio - CB radios were for Yanks and pussies, and I wouldn't have been able to hear anything, anyway!
Used to make me spew when the Perkins was singing her heart out in 3rd underdrive at 20kmh with 13 tonnes aboard - and a Cleveland Freight Lines Mack-Muncher road train pulling 50 tonnes in doubles, would overtake me doing 85kmh, UP the hills!
Things have changed a bit nowadays! A
young bloke wouldn't even drive what we drove happily!
Cheers - Ron.
FollowupID:
762386
Follow Up By: Madfisher - Wednesday, May 30, 2012 at 13:29
Wednesday, May 30, 2012 at 13:29
At work we have an Isuzu 600 Pan , which has a six speed synchro box, but 2nd is very scratchy and it changes a lot smoother with a double clutch. I was explaining this to a
young bloke who was hiring it last week, next thing he bolts back into the office and asks if he could have an auto. Compared to the old 1500 inters I drove in the 70s this truck is a revlation, so quite in 6th(2000rpm at 100) it is difficult to keep it on 100, and it will pick up speed on a moderate
hill in 6th.
You need a degree to operate the
seat.
Never had a go at a Joey hence my interest.
Cheers Pete
FollowupID:
762415
Follow Up By: Axle - Wednesday, May 30, 2012 at 18:19
Wednesday, May 30, 2012 at 18:19
Pete, those old 1500 inters where good in their day!,... Imagine trying to run one now on the current fuel prices!...LOL.
Cheers Axle.
FollowupID:
762439
Follow Up By: Madfisher - Wednesday, May 30, 2012 at 19:23
Wednesday, May 30, 2012 at 19:23
LOL Axle , nearly 40 years ago, but I remember they did 8 to 12 mpg depending on gearing and body. Six hours to get from
Sydney to
bathurst. 3rd gear up any slight incline ans we only had furniture on. I remember Dawsons got a 1500 Acco with a two speed diff and a header and we thought that was great.Then they got a bedford and grafted a 308 v8 in, for its day it flew.
They also ran a 1600 long nose that was pretty
well flat at 50mph.
Cheers Pete
FollowupID:
762447