Planetary gearhead vs timing belts

[Swede]

I have a number of very nice Thomson and Bayside planetary gearboxes on hand. Most are 5:1 or 10:1. I've noticed that guys tend to use timing belts to gear their leadscrews rather than gearboxes.

In planning for my next mill, I am going to be faced with a very heavy Z, and the leadscrew will require gearing for a mechanical advantage. What are the pros and cons of using a planetary gear vs. a timing belt? I'd like to put some of these gearheads to work.

Thanks!

[Al_The_Man]

I have used both methods and some of the things I have observed is under 5:1 the timing belt is usually OK, it has less inertia and is cost effective, once you start going up in ratio the tendency is to use aluminum timing belt pulleys to keep the inertia down, which tend to wear over time, the tooth starts wearing narrower causing backlash, also pulley wheel dia. is a consideration.
At 10:1, and definately if over 10:1, I would start to look at precision planetary gearbox, they would be more compact and the inertia number starts to overlap, You have to make sure the torque output is up to the demands of the system. Also the input rpm required to achieve the desired feedrate.
Al

[ViperTX]

How much backlash do those planetary gearheads have?

[aghobby]

Precision gearbox have habitually less than 15 arc/min of backlass. More $$ you spend , less backlash you gest, some mfg. have 0 backlash gearbox that are very expensive. I have used gearbox from GEM and from CGI with very good results. With a 15 arc/min backlash, convert in degrees this give a .25 deg of backlash, add a 3" pinion gear to go with a rack, you will get a total backlash of .002", that is good enough for me, list price for a 5/1 nema 34 std. from CGI is 380$.

Hope this help,
Alex

[Swede]

Thanks for the replies fellas. My better gearboxes are Thomson Nema-Tru's with 0.22 degrees of backlash. With a 1000 line encoder, this is better than a single line on the encoder disk, for all intents, at least with my machine, 0 backlash.

I need to do some math and figure out what the top speed will be with my system. The inertia will certainly go up, and the ability to accelerate rapidly will drop, but of the three axes, if I was going to hobble one of them, I'd pick the Z. One other benefit of a gearbox is that they can be easily retrofitted to a currently direct-drive axis. No elaborate mods needed, just detach the servo, add the gearbox, modify/change the coupler, and go for it. Much easier than retrofitting for a timing belt.

I wish my Thomsons were more like 5:1... 10:1 may be too slow. This is going to be for my new Gilman box spindle which is heavier than I thought. :frown: