Re: Novice Timing Belt question

Robert,

Some good questions. Here are hopefully a few good answers!

First, the belt length is shorter with the single length/fixed end
technique, which means less cost, less belt to stretch.

Next, the alignment of three rollers (in close proximity to each
other) and two belt end fixings is often much easier to accomplish
than two widely separated pulleys, one of which must allow for belt
tensioning. The fixed belt ends are easy to tension.

With two widely spaced pulleys there is more likelihood of cyclic
interaction, if they are not in perfect alignment and accurately
round, though this is also true of any system which uses round
driving elements (which pretty much means all of the low end
techniques we use to move machine slides). What is different here is
that the two belt lengths will oscillate in response to any cyclic
variation, and set up vibrations, and pose resonance issues...

By laying the belt flat on a surface, these cyclic perturbations are
damped, and the single length minimises their occurance in the first
place.

With a full belt, shielding of the belt grooves from swarf, dust,
cuttings, etc. is harder. One side of the belt, at a minimum, will be
exposed. Turning thewhole thing 90 degrees allows for all the aobve
to fall through, at the expense of more difficult bearing mounting.
The single belt is usually laid flat, grooves down, and the idler
rollers allow it to hump up over the driver pulley. In this way, the
grooves are always in a self cleaning position, and the only
shielding is again the very small space of the three rollers, which
is very simple to do.

Hope this helps,

Ballendo

P.S. There are a huge number of machines using single flat belt
drives commercially. Many of these are high speed, pick and place
type machines; some huge for use in automated warehouses. One mfr. in
the "router" arena is Precix, and a source of purpose designed
belting and end clamps is Brecoflex. As I usually do, I will again
stress a STRONG preference for HTD style belts (or their many mfr
proprietary derivatives) over the XL trapezoid type.

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "robertokx" <robert.okx@s...>
wrote:

> I have pretty much given up on trying to economically source a ball
> screw for the long axis and am looking at timing belts.
>
> From the CNC machines on the web it appears that the most popular
> configuration to use is the fixed belt method ie "....the driving
> axis, idle rollers, and platform proceed as a unit along a length

of

> belt that's held fast at both ends. Belt and drive sprocket impel

the

> load while linear bearings support it"
>
> My question is why isnt the method where the belt is stretched

around

> a driving and idler sprocket used more often? ie "....The driving
> pulley pulley and idler pulley of equal diameter are offset. The
> load is linked to the belt and moved parallel to the span between

the

> pulleys. This system contains a platform or slider that moves on
> linear bearings"
>
> For diagrams of both systems see:
> http://apps.mectrol.com/applications/doc/linear_day2.pdf
>
> The reason I ask this question is that with the fixed belt method

the

> weight associated with the motor, idler rollers etc must all make

the

> trip around the table meaning bigger motors may be required, more
> inertia etc and also the fixed belt method appears to require more
> parts. I'm assuming that there are advantages of the fixed belt
> system over the closed belt which more than offset these
> disadvantages.
>
> Thanks for any responses, in advance.
>
> Robert