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engineer1969 (Marine/Ocean) (OP)
28 Nov 06 10:22
Could someone let me know what exactly is meant by "number of starts" when it comes to picking a worm and wheel.

Cheers
Helpful Member!  Philrock (Mechanical)
28 Nov 06 10:58
A standard screw or bolt has 1 start.  Let’s talk about a ¼-20 thread.  Let’s keep the shape of the thread’s cross section the same but make the helix angle steeper so there are 10 threads per inch instead of 20.  We now have a single thread wrapped around a cylinder.  Now make a copy of that thread and place it 1/20th inch away, axially, from the 1st thread.  We now have 2 separate but intertwined threads.  This is a 2-start screw.  Its pitch is 1/20th inch, and its lead is 1/10th inch.

The above illustration applies to the worm.  The gear must have a helix angle that corresponds with that of the mating worm.

The number of leads used on worms typically ranges from 1 to 6, though more are possible.

engineer1969 (Marine/Ocean) (OP)
28 Nov 06 11:40
Is there any advantage to using 2 or more starts? From my calculations I need 18 teeth on the wheel with a one start worm. Does this mean I would therefore need 36 for a two start worm? I fail to see the advantage of this as it will just take up more room!

Cheers
eromlignod (Mechanical)
28 Nov 06 12:42
Basically it only affects the lead of the worm, which affects the reduction ratio and thus the mechanical advantage and speed.  A two-start worm, for example, would have twice the lead of a one-start worm of the same pitch.  So when mated with a gear of similar diameter it would have half the mechanical advantage and the wormgear would turn twice as fast.  Similarly, for a four-start worm, the reduction is changed by a factor of four.

The additional starts do NOT, however, increase the strength of the assembly, as some mechanics believe.  There is NOT more than one thread meshed with the gear at a time to share the load.  There is still a single tooth transmitting torque, just like a single-start arrangement and the gear tooth strength still depends basically on its pitch.  Only the helix angle of the teeth is different.

Don
Kansas City
BobM3 (Mechanical)
29 Nov 06 9:59
For the same ratio, the two start worm would run more efficiently (less friction torque) due to the larger lead angle.
Spurs (Mechanical)
21 Dec 06 22:49
Engineer1969

The advantage of using 2 starts instead of 1 is that the worm becomes larger.  This is especially handy if you are designing a drive with high efficiencies where you need to have a worm with a high lead angle (say over 15 degrees but cannot manufacture it if the worm becomes too small.

As the lead angle increases, the pitch diameter of the worm drops based on the following equation:

Worm Pitch Diameter = Normal Module x # of starts/sin(Lead Angle)

So if you want high efficiency, choose higher lead angle and if the worm is too small, add thread starts.

You are correct however that to keep the same overall ratio, you must proportionately increase the number of teeth on the gear.

www.webgearservices.com
eromlignod (Mechanical)
22 Dec 06 10:39
The diameter of the worm is not really related to the number of starts.  The worm diameter can be most any reasonable size providing the gear is machined to match. I have had custom pairs machined like this many times.

The gear catalogs that I have offer one-, two- and four-start worms for the same pitch all with the same worm diameter.

Don
Kansas City
Spurs (Mechanical)
22 Dec 06 11:21
Don

If you look at the formula in my previous post, you will see that for a given normal module and lead angle, the number of starts does affect the size.

The catalogues that you refer to are adjusting the lead angle and Normal Module in order to maintain a size.

Yes the gear has to be designed to match.

www.webgearservices.com
gearguru (Automotive)
22 Dec 06 16:55
eromlignod is right, read his post carefully:
"The worm diameter can be most any reasonable size providing the gear is machined to match."
Spurs (Mechanical)
23 Dec 06 13:39
Gearguru

Engineer1969 asked the question about using 2 starts instead of 1 start.

This therefore presumes that one has a fixed ratio in mind, and probably normal module, a helix angle and pressure angle.

Therefore my response on size is correct given the nature of the question.  If the design has a small normal module, and a high helix angle, then the major diofference between a 2 start vs 1 start is size.

This is exactly what the pitch diameter equation also tells us.

Yes, once can make other adjustments in the helix angle and normal module to make the worm whatever you want, but now you are talking about more changes than just the number of starts.
gearguru (Automotive)
23 Dec 06 14:19
We also can presume that the center distance is fixed; then we run in some more problems.
Merry Christmas, gear folks!
Helpful Member!  Warpspeed (Automotive)
31 Dec 06 18:50
It can also really comes down to the required ratio.

If the ratio was 100:1, a single start worm with a 100 tooth gear would be the logical choice.

But what if the required ratio was only 5:1 ?  Cutting a five tooth gear might be rather a problem. But a four start worm with a 20 tooth gear might be a lot more practical.

When the worm has a large number of starts, it begins to look more like a helical gear than a worm.

How about a 20 start worm driving a 20 tooth helical gear ? Then you have just a pair of crossed helical gears (or crossed worms???)
Spurs (Mechanical)
31 Dec 06 20:05
A cyclindrical worm mating with a helical gear is a crossed axis helical gear system regardless of the number of starts on the worm.

The point I was trying to make on size can be summed up in this example:

Goal:
Create a 16 to 1 drive with an efficiency of greater than 60% using a 1.0 Normal Module and 20 degree normal pressure angle:

For the selection of materials and lubrication conditions, it is anticipated that the coefficient of friction is .15

Possible Solutions

To achieve a minimum of 60% drive efficiency, we will need to use a lead angle on the worm of at least 15 degrees.

Therefore for the worm: the standard pitch diameter would be 3.236 mm, and the root diameter would be approximately 0.226 mm.  This is not very manufacturable.

One possible solution is to use a 2 start worm with a 32 tooth gear.  In order to obtain the same drive efficiency, we must use the same lead angle on the worm, therefore the 2 start worm will now have a pitch diamter of 6.472 mm and a root diameter of approximately 3.462 mm.  This may be a more realistic solution.

www.webgearservices.com

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