Timing belt a non-standard pitch?
11-10-2006, 05:37 PM
#31
Rennlist Member
Mike, if you look at it from an "ease-of-manufacturing" perspective, there is a perfectly good reason why the holes are skewed. The basic piece was probably manufactured as a regular-shaped part; that is, there are three arms that are identically spaced around the hub at 120° intervals. Then the last two manufacturing steps(aside from breaking edges, tumbling for finish, etc) were to broach the keyway and drill/tap the holes. More likely they were drilled, broached then tapped, because threaded holes are poor location points. So, there were several steps in fabrication where the only orientation that mattered was front/back.
Back in the days of manually-indexing rotary tables, etc. it probably made a difference. 30 years later, with CNC being the norm, it's easy to look back and say it would be just as easy to make the arms of the spider asymmetrical to match the final hole pattern. It looks like the angles are something like 128°, 118°, and 114°. Only one machine had to be set up with those oddball angles, the one that drilled the holes. If they had designed the arms to be at the same angles, then the machine that made the rough shape would have needed to be set up with these oddball angles. By using 120° they were probably able to use built-in stops in the rotary device they used for the machining, saving a lot of money on setup time and fewer scrap parts due to better repeatability.
Back in the days of manually-indexing rotary tables, etc. it probably made a difference. 30 years later, with CNC being the norm, it's easy to look back and say it would be just as easy to make the arms of the spider asymmetrical to match the final hole pattern. It looks like the angles are something like 128°, 118°, and 114°. Only one machine had to be set up with those oddball angles, the one that drilled the holes. If they had designed the arms to be at the same angles, then the machine that made the rough shape would have needed to be set up with these oddball angles. By using 120° they were probably able to use built-in stops in the rotary device they used for the machining, saving a lot of money on setup time and fewer scrap parts due to better repeatability.
06-28-2007, 03:27 PM
#32
Inventor
Rennlist Member
Rennlist Member
I stumbled across these timing belt tables (and this thread) while looking for a longer belt for the 928 - (Dayco) (Gates). Unfortunately, no other belt is long enough in the rather thin 1" 928 size. I'd really like to find a Continental or other european maker's list.
If one were to make custom pulleys, you could use a standard HTD tooth pitch with more teeth, instead of the funky 3/8"/9.525mm/L pitch converted to HTD. Then it would be easy to order custom belts, in kevlar or whatever.
Think we have a long belt at 1" x 81-1/8" x 219 teeth? The Audi A8 has a 1-3/16" x 94-3/16" x 299 teeth belt!
If one were to make custom pulleys, you could use a standard HTD tooth pitch with more teeth, instead of the funky 3/8"/9.525mm/L pitch converted to HTD. Then it would be easy to order custom belts, in kevlar or whatever.
Think we have a long belt at 1" x 81-1/8" x 219 teeth? The Audi A8 has a 1-3/16" x 94-3/16" x 299 teeth belt!
06-28-2007, 03:42 PM
#33
Inventor
Rennlist Member
Rennlist Member
If you are making new pulleys, then why not add some active damping in the pulleys, too. 
Here's some awesome texts I found a while ago concerning valve train vibration (right-click, Save Target As...):
http://www.ricardo.com/download/pdf/...ming_drive.pdf
http://www.ricardo.com/download/pdf/valdyn_qohc.pdf
Here's some awesome texts I found a while ago concerning valve train vibration (right-click, Save Target As...):
http://www.ricardo.com/download/pdf/...ming_drive.pdf
http://www.ricardo.com/download/pdf/valdyn_qohc.pdf
