RE: [CAD_CAM_EDM_DRO] Digest Number 1138
Posted by
Rose, Gary
on 2001-02-19 08:10:10 UTC
Les,
The positional accuracy numbers you are referring to appear to be from an
example in the Gates catalog. Looking at the numbers carefully, though, the
2mm belt was tested with a 12 groove pulley driving a 40 groove pulley (over
a 3 to 1 ratio) whereas the 3mm belt was driven 1:1 with 20 groove pulleys.
This will account for "some" of the discrepancy in the numbers, not all
however. Notice too, that the application that the 2mm belt was quoted for
was "motion transfer" - that's what you want. The 3mm belt application was
"light load" - this is an apples to oranges comparison. It would have been
more helpful if Gates decided to use COMPARABLE examples for comparison -
but you take what you can get.
The reason this is important is that the belt material, and tensile members
will have an effect on the belt stretch, this will affect backlash. The
quality of the pulley will have an effect - machined pulleys will generally
be more accurate (positional accuracy as well as tooth clearance) than
molded ones - less backlash. As well, the installed tension is generally
higher for registration drives than for power transfer - this helps with the
backlash situation. There is no easy answer or table that you can find
values for belt accuracy. It depends on the belt material, tensile members,
tension, center distance, pulley accuracy, etc. They don't tell you any of
this in the example numbers. You can get belts with different materials
that will assist in achieving greater accuracy - but if you're not ordering
directly from Gates forget it. My personal opinion is use the best pulleys
you can find, shortest centre distance, and highest installed tension that
you can live with - then test your system and see what you really get.
About the tooth jump issue, you are right about the weak function of width -
doesn't have much bearing. Tension does! We frequently cure marginal tooth
jump problems by cranking up the tension. This only goes so far though.
If you're interested, Gates has a white paper on belt accuracy you can check
out:
www.gates.com/facts/index.cfm?show=Engineering
look for document #289
Hope this helps
Gary
The positional accuracy numbers you are referring to appear to be from an
example in the Gates catalog. Looking at the numbers carefully, though, the
2mm belt was tested with a 12 groove pulley driving a 40 groove pulley (over
a 3 to 1 ratio) whereas the 3mm belt was driven 1:1 with 20 groove pulleys.
This will account for "some" of the discrepancy in the numbers, not all
however. Notice too, that the application that the 2mm belt was quoted for
was "motion transfer" - that's what you want. The 3mm belt application was
"light load" - this is an apples to oranges comparison. It would have been
more helpful if Gates decided to use COMPARABLE examples for comparison -
but you take what you can get.
The reason this is important is that the belt material, and tensile members
will have an effect on the belt stretch, this will affect backlash. The
quality of the pulley will have an effect - machined pulleys will generally
be more accurate (positional accuracy as well as tooth clearance) than
molded ones - less backlash. As well, the installed tension is generally
higher for registration drives than for power transfer - this helps with the
backlash situation. There is no easy answer or table that you can find
values for belt accuracy. It depends on the belt material, tensile members,
tension, center distance, pulley accuracy, etc. They don't tell you any of
this in the example numbers. You can get belts with different materials
that will assist in achieving greater accuracy - but if you're not ordering
directly from Gates forget it. My personal opinion is use the best pulleys
you can find, shortest centre distance, and highest installed tension that
you can live with - then test your system and see what you really get.
About the tooth jump issue, you are right about the weak function of width -
doesn't have much bearing. Tension does! We frequently cure marginal tooth
jump problems by cranking up the tension. This only goes so far though.
If you're interested, Gates has a white paper on belt accuracy you can check
out:
www.gates.com/facts/index.cfm?show=Engineering
look for document #289
Hope this helps
Gary
> Date: Sun, 18 Feb 2001 13:22:26 -0500
> From: "Les Watts" <leswatts@...>
> Subject: ServoTiming belt selection
>
> Well I have spent part of today selecting timing belts as I want
> to get them ordered first thing tommorrow. I can have the
> machine actually
> moving as soon as I get the belts and pulleys.
> I will have to bore some of the pulleys but no big deal.
>
> I am using the modified curvilinear GT-2 belts due to their
> supposed improved registration and lower backlash compared
> to a trapezoidal timing belt.
>
> I found the GT-2 3 mm belts 9mm wide will jump teeth
> at 640 oz in with about a 1" pulley with rec
> omended tension.
>
> GT-2 5 mm will jump teeth at 4800 oz in with similar setup.
>
> 2 mm jumps at only 128 oz in so is not a consideration.
>
>
> As previously set up 3 mm gives a max position error of .012".
>
> 2 mm gives max position error of .0002"!
>
> 5 mm is not listed.
>
> This is from the Stock Drive Products web site at:
> http://www.sdp-si.com/herb/spk/sdp-si/D260/D260cat.htm#S10
>
> I think They may have a decimal place off somewhere in the
> positional accuracy data. 3mm 60 times poorer than 2mm??!!
>
> I could not access the gates rubber design program to check this. Does
> anyone have their cd so I could get a verification
> of these numbers?
>
> I am using 2:1 reduction with a 1" small pulley to insure
> at least 6 tooth engagement. Width is 9mm to 15 mm but
> I think jump torque is a weak function of width.Suprisingly
> it is shown as a
> very weak function of tension. It may be a strong function of
> positional
> accuracy though.
>
> Center to Center distance is about 5".
>
> Thanks,
>
> Les
>
>
> Leslie Watts
> L M Watts Furniture
> Tiger, Georgia USA
> http://www.rabun.net/~leswatts/wattsfurniturewp.html
>
>
>