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Straight vs Helical


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#1 Guess-Works.com

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Posted 12 April 2005 - 02:23 PM

Right, I've bit my tongue for long enough.... this is just one of those things which realy winds me up....

Which is stronger, Helical cut gears or Straight cut...

First of all I think we should defined what is meant by strength, IMO, this is the ability to transfer power without failure ( breaking ) from one gear to another ( input to output )

So...

A Helical cut gear is basically the same as a straight cut gear but on an angle ( pitch ) This pitch means there is a small overall increase in surface area, and also more than one tooth is in contact at any one time, compared to a Straight cut gear.

But...

On a helical gear only on a small section of a tooth is ever in contact, and at no time is any one tooth fully engaged, unlike a Straight cut where the faces are head on to each other and hence fully engaged.


Here's an amusing analogy...

Smack your forehead with the palm of you hand, hurts don’t it... This is similar to what happens when a SC gear engages.. If you repeatedly do this, instead of sounding like slap-slap-slap you would actually get a whine ( not from the person's head you are slapping !! lol )

helical cut gears are more like smacking your head with you hand, but at an angle, you hand sort of rolls across your forehead, the slap, is quieter, and the force is applied across the forehead.....

Which hurts more ?

:-

Edited by GuessWorks, 12 April 2005 - 02:24 PM.


#2 jimbo_edmondo

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Posted 12 April 2005 - 04:46 PM

They both hurt, can't really tell which is more!!!!
What do the race minis use?

#3 dklawson

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Posted 12 April 2005 - 04:56 PM

I've posted my comments on the other thread so I won't repeat them all here. See:
http://www.theminifo...opic=7305&st=15

Even with straight cut gears you don't have the full width of the tooth in contact, in-spite-of what you think. Well... they may touch all the way across the face of the tooth, but that only happens when the gear tooth surfaces deform (elastically) such that they are in line-on-line contact across the face.

Be careful with the terminology. The "pitch" of a gear is not the angle of its threads. Pitch is a term associated with the tooth size and spacing. What helicals have are teeth on a... helix. The helix is added in a complex manner to the pitch in an attempt to maintain the involute tooth profile. As you implied, this results in points of contact that "move" along the width of a gear tooth as it and its mate are rotated against each other. This point contact creates high Hertzian contact stresses. However, these stresses spread out quickly into the body and root of the tooth.

What does this mean? It means that helicals should in theory be able to carry more load for a given situation but their surfaces may fatigue (fret and chip) earlier than straight gear teeth. They will also produce axial loads due to the contact forces of the mating gear teeth. This moving contact patch also introduces friction which consumes part of the input power.

So it's a trade-off. Helicals are very quiet and strong but they may wear out sooner. Straights are (as you said) noisy and have a slightly lower load capacity. However, they are inexpensive to make and have minimal friction losses. I like quiet gears.

EDIT: Race Minis typically use straight gears and drop gears to minimize power loss.

#4 Bungle

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Posted 12 April 2005 - 04:56 PM

the hammer :-

#5 Bluemini

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Posted 12 April 2005 - 05:06 PM

In therory the helicals should be stronger because like said before, there is more area of contact on each tooth. But straight cuts transfer power better because there is no side load on the gear like there is on the helical ones.

#6 MiniMoi

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Posted 12 April 2005 - 05:11 PM

And DK has said it all...

#7 Purple Tom

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Posted 12 April 2005 - 05:36 PM

it fascinates me that there is so much more to these sort of things than you'd think. In the 50's when Mini gearboxes were invented would they have considered such things when building it? Or would it not have worried them? (that isn't a rhetorical question, i'm actually asking! :grin: )

Our 1955 Scammell Constructor has a straight cut 6 speed dog box. It was designed to tow military bulldozers, with a full ballasted laden train weight of nearly 50 tons. I think the thinking then was that straight cut gears were much stronger. I wish they'd considered our ears though! Scary stuff.

#8 Oldskoolbaby

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Posted 12 April 2005 - 07:17 PM

Personaly I think you should just look at the resuls of straight cut gears and ask yourself some questions

1. Could Matt Woods Turbo knock out 225bhp, as it reliably does, through a standard gearbox?

2. Could Carl Austin run his 400bhp twini up the strip with out 2 straight cut box's? Remember he does full blown starts!!! And finally

3. Could the awsome japanees Auto Delta 210 bhp 16v race car run without a straight cut box?

I think its quite obvious!!!!
If they were weaker, im sure years of motorsport development would have found this out.

#9 TimS

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Posted 12 April 2005 - 08:43 PM

they all run stright cut boxes due to the smaller power losses! and yes a stright cut box is engineered well enough to take it, but it still doesnt mean that the helical gears are weaker

#10 dklawson

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Posted 13 April 2005 - 01:30 AM

I've already spoken my piece about this topic but wanted to post this link for your review. Scroll down to Helical vs. Straight
http://www.baysidemo...5b?OpenDocument
and
http://www.mancheste...vs_sc_gears.htm

#11 Guess-Works.com

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Posted 13 April 2005 - 03:58 AM

G-Force Transmissions USA

Straight Away
In addition to the dog-ring gear engagement, G-Force offers a straight-cut gear set to replace the helical-cut gears found in a stock transmission. Helical gears work well in stock applications because they run quiet, but the downside is that there is less material in contact when compared to a straight-cut gear. "Helical gears are generally used for noise reduction, and that's the only reason the factories use them," stated Long. "The straight cut gears are stronger, but noisier, sometimes much noisier. The strength in straight-cut gears comes from the fact that you have full contact across the whole tooth. With a helical cut gear you only have a small point of contact, so you're only loading a fraction of the tooth at any given time. But that's what makes them quiet. The additional noise found with straight-cut gears comes from the gears actually slapping together. If you could break the noise down it would sound like slapping, but the repetitiveness makes it comes off as a whine." G-Force also employs gears with a coarser pitch on the teeth, which equates to stronger teeth. Lastly, you can upgrade to stronger shift forks for $250. Aside form having greater resistance to breakage, the stronger shift forks reduce flexing and provide more precise shifting.


I think for every for, there is an against... or is it the other way round :erm:

As for the guy in Manchester, I'm failry sure if you went to Jack Knight or Tran-X and said you gears are "crude". I think he'd end up in the ship Canal with concrete wellies., but he makes valid points about the lubrication and bearings, The design of the gearbox does not lend itself to sideways loading.

I'm a firm believer that at a certain level of power output there is a need to switch from Helical to Straight, otherwise straight cut boxes would not exists and all competition would be done using helical cut gears.

'spose I'm just attempting to define a level at which that is done.

#12 dklawson

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Posted 13 April 2005 - 12:50 PM

There is another reason there will always be straight cut gears and firms like JK would probably verify this if you asked them....

When you are making a small number of custom parts, be they gears or anything else, you want to design them so they are inexpensive to make as there is no economy-of-scale. Close ratio gear boxes would certainly fit the description of a low quantity, short-run item. It is a much more difficult task to design a well fitted/mated pair of helical gears than a simple spur gear. The grinding machines have to be programmed (or have custom driving cams made) to produce a very complicated profile on helicals. Spur gears can be hobbed into rough shape, deburred, and placed into operation... a much less expensive proposition.

The article you posted from G-Force doesn't surprise me. While I was searching for information on this topic I found a link where email from ZF was posted (sorry... didn't save the link). Their reply was that all their gearboxes used helicals for their higher strength and smooth operation. I would expect any firm to give only positive reasons for their tooth profile choice. I'd argue about G-Force's comment about single point of contact with helicals. Keep in mind that you actually have multiple teeth in contact with helicals.

#13 mighty_mini

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Posted 14 April 2005 - 12:30 PM

Either way, ....... That Sound.... :tongue: (Thats as technical as im getting about it) LOL

#14 duds100

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Posted 14 April 2005 - 02:47 PM

Mine sound sweet because ive got the drops aswell, I get more power at the wheels, and the box hasnt blown up yet so I think they are ace. :P

#15 Oldskoolbaby

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Posted 14 April 2005 - 05:46 PM

Flappyplasicbits your having a complete laugh. There is no way Matt Wood could run 225BHP, YES 225, reliably through a helical box. If you think that its in the way theyre made, ie stronger, why do the expensive kits that supply 5 gears helically cut only take about 90bhp(reliably). Remembering that they have gone through exactly the same design criterior in terms of srengthening as the straight cut gears when being produced???




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