Re: Engaging Gears Perpendicular to Shaft Direction?
 

I don't see any "undeserved comments" in anything John posted. John has been making some very good points, and he was just responding to some attempts to knock down those points. He was trying to stick to the point of the thread, and has done a good job at following that. He explained his reasons. Don't be one of the people who try to drag good threads like this through the mud over some sentences and wording you didn't like.

Re: Engaging Gears Perpendicular to Shaft Direction?
 

I talked with Matt and some other people about a question I've had, but I thought I'd toss it up on CD for some more opinions and discussion. I am not anywhere near as skilled in engineering math as I'd like to be, so keep that in mind, please.

What I was wondering, essentially, is if there are clear benefits to one method of "swinging" a gear into place over another.

Matt's posted design uses this method:


The gear pivots about a point and swings into place such that the line between the center of the output gear and swing gear is perpendicular to the line between the center of the swing gear and the pivot.

After seeing Matt's design, and for no other reason that my irrational fear of chain, I began working on alternative methods of achieving the same "swing" transmission design. My design, in order to eliminate the chain, relies on this method:


The gear pivots about a point that is colinear with the center line between the swing and output gears. It maintains the same center-center distance between those gears with rigidity.

Matt showed concern that the later method of gear engagement was more prone to breaking teeth and other such bad things, but I can't understand why. I don't see how that's the case, but I trust that Matt probably has a better understanding of what's happening than I do, and was hoping someone could explain it in detail.

Re: How's this kluggie cut up?
 

This is probably an elementary concern, but the rule of thumb I remember being told about chains and sprockets is that it's best for the chain to engage at least 90* of the sprockets circumference to ensure it doesn't slip.

Is that a concern here?

Re: Engaging Gears Perpendicular to Shaft Direction?
 

First off, I'd like to say that my primary concerns with engaging gears along the direction of rotation as shown in method number 2 are that of dynamic forces. Assume that one (or both!) gears are rotating at a reasonable rate counterclockwise when they're trying to be engaged. Eek! You have a sizeable angle of motion where the gears are not at the pitch circle diameter, as well as dynamic forces opposing the motion. Outer edge and angle shown in red (circles and angle shown slightly bigger for clarity)




While there maybe be some benefit of one gears in the above case rotating clockwise, essentially "pulling" the gear... you still have "tip of tooth" engagement over it's travel. This scares me.

In the case where the arc path swings towards the center, the distance travelled in the "danger zone" of outer tooth edges is as short as possible. I think this is better.

I think it's important to realize too that this is a very non-traditional method in the first place.. anything I can to make this safer I'm going to do.

In addition, the last time that Miss Krass and I spoke, which may have changed, she was proposing that style number two not engage a single gear as shown in the drawing, but swing in-between two gears and function as an idler gear. I think this would have a dramatic increase in potential binding and would have immense tolerances requirements.

Anyway, that's my two cents. Anyone else care to share?

Matt

Re: Engaging Gears Perpendicular to Shaft Direction?
 

Just so there's no confusion about what we're talking about, I'll toss these screenshots of my own design into the mix so that people can see it in practice.

In high gear:


In low gear:


The idler gears swing between the input (blue shaft) and output (gold shaft) gears. They are fixed, however, relative to the output gears and remain engaged at all times to the output gears.

I agree, of course, that the "danger zone" may be greater for the method I used here, but I wonder how significant of a difference it really is. It may be fractions of a second, which may or may not be enough time to noticeably increase the potential for damage to occur.

Re: How's this kluggie cut up?
 

It is a pretty good rule of thumb to say that 90deg of wrap, but it is not written in stone.

With a tight chain, you can do less -- especially if you have more than 3 or 4 teeth engaged (for example you may be able to have fewer than 90 deg wrap with a sprocket with more than 20 teeth on it).

But, that said, I don't think this design necessarily breaks the 90 deg. rule. I believe with some clever routine of chain (and perhaps moving the drive/idler axle higher) you can get the 90 deg. wrap angle that will let you sleep at nights.

Another exercise I will leave for the student...

Joe J.

I STILL don't like it...
 

I know that a lot of time and energy has gone into this type of radially engagement of gears as a means of shifting gears, but I have to tell you all that it is creeping me out pretty bad.

This is not a really good solution. Engaging drive dogs is one thing, involute gears are another still.

For all this, I keep going back to the shifter on the drill itself.

If you really want to design a good rookie solution, crack that nut. Beleive me, this is a much more doable solution for 99% of the rookies you are trying to help.

Joe J.