LT95 Overdrive

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Originally Posted by Brian Hjelm

Theory vs. practice raises its ugly head again and again in engineering problems. Spur gears last better and longer in rough use. Helical gears are quieter hence their use in road going vehicles particularly passenger carrying vehicles.

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We will have to agree to disagree.

The trend for many years has been to build smaller gearboxes that transmit increased power and torque. You will not find any large manufacturer of gearboxes today that use spur gears.

In a previous life I worked for the manufacturer of the largest gears in Australia - the largest in my time weighed 200 tonnes. They have since installed larger gear cutters.

We also designed and manufactured the largest sugar mills in the world, which were driven by multiple large kW steam turbines. Helical gears were used for the transmitted power. Smaller sugar mills use long addendum spur gears to connect the crushing rolls together, but this is because the rolls wear and the operating centre distance between rolls has to be reduced (the large mills have separate cardan shafts to each roll instead of gears).

More recently I have been involved with the design of grinding mills. We produced gear driven mills with installed power of 4.6 MegaWatt. For larger mills over 7 MegaWatt, gears were not used , instead wraparound motors were more practical. The gearwheels for the 4.6 MW drives were too large for manufacture in Australia, but the pinions were made here. No one in their right mind would have contemplated using spur gears.

Current gear rating theory is incredibly complex. Practice has validated the theory over many years. There is no possible way that spur gears can be designed as small as helical gears for the same rating.

MegaWatts:eek:, that's a fuggin big mill:eek::eek::eek:

when you start working with herringbone gears and get the gear to cut them someone come and talk to me Im dreaming of rebuilding the R380 with a full compliment of herringbones and then replacing the syncros with dogs.

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Originally Posted by Bush65

We will have to agree to disagree.

The trend for many years has been to build smaller gearboxes that transmit increased power and torque. You will not find any large manufacturer of gearboxes today that use spur gears.
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Current gear rating theory is incredibly complex. Practice has validated the theory over many years. There is no possible way that spur gears can be designed as small as helical gears for the same rating.

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The advantage of spur gears over helical gears is two fold - because they do not have significant sliding contact, they are a lot less demanding as far as lubrication goes (and stand running without oil better) and they do not impose side thrust, with all the implications that has for case and bearing design. This does not mean they last better and longer in rough use, unless you include poor design and poor lubrication in "rough use", which of course, you can.

As Dave points out, much better than helical gears are herringbone gears - all the advantages of helical gears without the side thrust. Only problem is making them. An interesting side point here - how many of you know that the double chevron used by Citroen as a logo comes from herringbone gears? Andre Citroen built up an engineering business during WW1 making herringbone gears for various bits of machinery, and then branched out into car manufacture after peace broke out. Some of the early cars had herringbone gears (including crown wheel and pinions - I have seen one!), but market realities soon put a stop to that.

John

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Originally Posted by JDNSW

Some of the early cars had herringbone gears (including crown wheel and pinions - I have seen one!), but market realities soon put a stop to that.

John

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AEC trucks & buses own double reduction differentials had herringbone gears into the 1970's well after their "merger" with Leyland.

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Originally Posted by Brian Hjelm

AEC trucks & buses own double reduction differentials had herringbone gears into the 1970's well after their "merger" with Leyland.

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Yes, but they were not selling cars in the low cost end of the market like Citroen were in the 1920s. Actually, as you point out some people have used herringbone gears, and it is obviously possible to make them, so it is surprising that they are not more widely used, particularly with the emphasis on weight saving these days - if you do not have to cater to the end thrust of helical gears you can save a significant amount of weight on both the case and bearings, particularly where a lot of power is being transmitted.

John

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Originally Posted by isuzurover

4BD1 is about 250Nm, 4BD1T is about 320Nm in standard spec. Probably wouldn't be hard to get one to produce more than 380Nm though.

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Currently knocking out over 500Nm:twisted:

I just had my worst tank of fuel in a long time. I ran out (about 75L) at 600km.
This was all urban work, shifting house with a trailer in tow and lighting up tyres at each intersection.:D

Fuel gauge was on holiday, fuel light came on and it coughed to a stop 200m later.

The disadvantage of herring bone gears is that most gear cutters (except Sunderland) require space in the centre of the V for cutter runout.

High performance gears must be cut to a very high accuracy (profile and pitch). Gear grinding machines are required obtain the accuracy. The allowable tolerances for the higher accuracy grades use in gear rating are smaller than the average machine shop will achieve machining common parts like shafts.

The gear cutter/grinder has to compensate for the change in tooth profile as the tooth flexes and as the pinion bends and twists under load.

The increased face width of herring bone gears lead to greater deflection of the pinion.

The grinding mills I mentioned in the previous post have arrays of infra red sensors than measure the temperature across the width of the gear teeth close to the mesh. Here we are looking for changes in the tooth load distribution because of deflection.

If anyone is near Orange (NSW), Cadia gold mine run public tours. If you have a chance to look at the grinding mills, they are impressive machines - particularly the SAG (semi autogenous grinding) mill, with the wrap around motor.

Grinding mills rotate in plain bearings (white metal or aluminium) and only have a lower bearing shell. They typically use 4 by 500 tonne jacking cylinders to lift one end of the mill for bearing maintenance.

End thrust from helical gears is a simple engineering problem.

I guess this thread has well and truely been hijacked from issues related to overdrives on the LT95 with a Isuzu diesel to a discussion on different types of gears.

I was hoping for more info on the pros and cons and general discussion of the o/d in the configuration would come out.

Garry

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Originally Posted by garrycol

I guess this thread has well and truely been hijacked from issues related to overdrives on the LT95 with a Isuzu diesel to a discussion on different types of gears.

I was hoping for more info on the pros and cons and general discussion of the o/d in the configuration would come out.

Garry

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ME TOO!!!:mad: