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Straight vs Helical


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#16 dklawson

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Posted 14 April 2005 - 09:32 PM

I know that this is beating a dead horse but I'll make one more post to this thread.

The arguments against helicals have continually contained comments like "the racers use" and some comment about power. Anecdotal evidence like that isn't supported by engineering fact. I don't disagree that racers would want straight gears, nor would I make the argument that standard helical gears from Rover would be strong enough for the most punched up A-series engines.

However, the fact remains that when manufactured to equal degrees of quality and with equal materials, the helical profile will be stronger. I finally dug out one of my texts on machine design. Quoting from "Standard Handbook of Machine Design" by Shigley & Mischke, McGraw-Hill, ISBN 0-07-056892-8, section 35.3-2. (Without permission).

"Load Capacity:
As a result of increased total area of tooth contact available, the load capacity of helical gears is generally higher than that for equivalent spur gears. The reason for this is obvious when we consider the contact line comparison which figure 35-3 shows. The most critical load condition...."

It goes on for a paragraph about the topic.

This is a definitive text. Think about this and ask some of your friends in the navy what type of gears are hooked up to the turbines on their ships. They won't be spur gears.

#17 Oldskoolbaby

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Posted 14 April 2005 - 09:42 PM

That probably cause they have to run silent at one stage or another. Can u imagine trying to get to sleep with straight cut gogs the size of lorry wheels spinning. Now that is what ill call staight cut whine:)

#18 Dan

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Posted 14 April 2005 - 10:05 PM

why do the expensive kits that supply 5 gears helically cut only take about 90bhp(reliably)


Because ALL (and I really mean ALL) the 'expensive kits' you describe use the standard Rover laygear and first to top speed gears and in fact all other standard parts. The only extra is the overdrive gearset which has been specifically designed to work with this standard set. The Rover gearset is designed to cope safely with the output of a standard engine. The reason the power level it can handle drops slightly when you add the fifth speed is because the fifth is an overdrive gear which increases the effective loading seen by many parts inside.

'Racers' do like to use straight cut gears of course as has been said. The main reason in most race series that this occurs is that it is far easier to swap speed gears in a straight cut box. It is possible using spur gears to get many different ratios using a common laygear. What I mean by this is that given one driving gear it is possible to manufacture several different driven gears of different ratios which will safely mesh with it. Like the Jack Knight drop gear sets for Mini's where several ratios are available using a common idler gear and two or three different primary and input gears. This means that not only do racers get a whole box full of different cogs to play with so that they can match their gearbox to each circuit, but it saves many thousands of pounds in gearset design and parts. And it makes it much quicker to strip and reconfigure the box.
This is not possible with helical gears.

Oh, and straight cut gears don't have to whine if you have the budget of a navy to spend on cutting and maintaining them.

Edited by Dan, 14 April 2005 - 10:06 PM.


#19 Oldskoolbaby

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Posted 14 April 2005 - 10:26 PM

Ok matey, I rest my case on the five speed gearbox, I thought that they used different sets of gears so ill bury myself a hole and hide in it.

I was only joking about the navy though and i do know that in certain circumstances they are quiet.




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