Screw Threads and Multi-Start Heads

What do you consider efficiency of the screw? The multi start screw has a higher pitch angle so it will thread faster, but I don't know if you would call that more efficient.

Except of faster screwing , is another advantage for multi-start head threads ?

It is more efficient in that you are using wasted space for more holding power.

The thread pitch is as such that it requires fewer turns of the screw, nut, bottle cap, or lead screw to travel a given distance. Because of this there is vacant unused space between the first set of threads. This space is used for additional threads resulting in increased strength for higher load capabilities.

It becomes less efficient in the respect that when you pass a certain pitch (sorry I don't know the angle) that the thread will not hold position on its own. If you are using it as a lead screw driven by a motor then it is not a problem. If you are wanting to use it to secure a bottle cap then it just wont work. I have made threads for a robot actuator that used such a thread. To look at it it looked like a normal screw because of the multi-start threads. But when you placed the nut on it and held it vertically and gave the nut a gentle spin it would travel down the full length of the shaft without stopping.

I'm not sure I can agree with The Mechanic........

Whether a thread is designated single, double, or triple, the pitch remains the same. I don't believe there is any increased strength either. In truth, I think the load capability decreases as the number of leads increase due to loss of mechanical advantage.

As far as the advantages the multi-lead screws goes, they simply advance further with each revolution.

I wasn't sure enough myself to post an answer to Mechanic, but what you wrote seems to be pure common sense to me. A GA for that.

The only advantage that I see is the fact that you can move the nut farther for one revolution.....maybe its slightly more stable?

Surely therefore the pitch angle increases?

Yes, that's the advantage, to move the nut further with a revolution.

The Pitch, being the distance between threads, stays the same, the Lead, being the distance each thread advances as it winds around the shaft, increases. ie- an 8 pitch (8 threads per inch) single start thread has a pitch of .125" and a lead of .125". An 8 pitch 2 start thread has a pitch of .125" and a lead of .250". A 3 start will have a pitch of .125" and a lead of .375". and so forth.

The pitch has no angle, it is only the distance between threads. The lead angle changes with the diameter, pitch, and number of starts.

Maybe this will help.

Pitch is measured from any given point on a thread to the same point on the next thread. Lead is the distance traveled with one rotation of the nut or bolt. A single lead 1"-8tpi bolt will travel 1/8" per revolution. A twin lead 1"-8tpi bolt will travel 2/8" or 1/4" per revolution. To make that thread you will have to set the machine up for a 1"-4 tpi but machine it to the 1"-8 thread depth unless otherwise called out. This way you can create what appears at first glance as a 1"-8 thread.

I may have misstated what I meant by increasing the load capabilities. I was referring to a multi start thread having greater capabilities than a single start of the same lead and thread depth but not the same pitch. AS I pointed out before, "It becomes less efficient in the respect that when you pass a certain pitch (sorry I don't know the angle) that the thread will not hold position on its own." I left it for you to assume that as the lead distance increases the overall holding or clamping force decreases. Just as you would interpret any other problem concerning torque.

When there isn't much material, (small part or very thin wall) that won't allow for a large/deep thread, ie 6 or 8 TPI, but quicker travel is required, Multi-start threads can be the answer. Typically used in a dynamic mood where clamping force isn't a issue. But there's always Loctite.

(One design I've seen was an 64p, 8-start! Marketing loved it, Manufacturing was OK with it, QA & Assembly hated it...$$$ for gaging and did you ever have to get a 8 start thread started? Really takes some doing!!!)

I can talk in terms of gears (worm) where the multistart are more used (and I am comfortable with )

The efficiency of transmission in an worm is defined as

η = tan α / tan (Φ+α) , where α is the inclination angle and Φ is the friction angle.

and

η_rev = tan(α-Φ) / tan α for reverse drive (wheel driving the shaft)

α_{single start} > α_{multi start}

and thus

n is more for multistart than for single start.

In fact in worms the η may just reach about 0.5 for the single start and the η_rev is negative (ie the gear can not drive the worm shaft)

Whereas for multistart, you can reach a drive efficiency of approx 0.9 or above but losing the η_rev which now becomes positive and hence the gear now can drive the worm. So you need additional braking/ locking arrangement for these.

I think you were looking from this angle ?

BTW- this same logic holds for the normal fastners too.