some rack questions.

[blazedforever]

1.) What are the benefits of using steel gear rack vs brass? beneficial property's of brass racks?

2.) What is the best y axis mounting configuration, and do most people Bolt through the side of the rack or clamp the rack into place?

3.) Rack module measurement, what is being measured?

4.) Is there anyone using helical R&P should i choose this over the spur gear ?

Any additional information on optimal Rack and Pinion setups will be appreciated.

[wildwestpat]

Hi

Brass v Steel - this is a question of wear and power or shock loading. Often brass and steel are used together one on the rack the other as the pinion to reduce friction.

The DP of the gearing has to be established and held over the service interval. For this reason the rack should be firmly bedded in the plane parallel to the rack linear motion. However it is often more convenient to clamp the rack but this has to be adjusted and held in service for correct mesh.

The module measurements are the same as for ordinary gears.


Helical R&P will give smoother progression for the same reason as helical gears are prefered over straight. This is because the pressure angle on each tooth overlaps with that of the adjacent teeth. Due to the expense this form of R&P is limited to precision drives.

I guess from the questions this is for a mill gantry or table drive in which case rack gearing may not be your best option. This is because of the slight variations tooth to tooth in translating rotary to linear progression. This will either exercise the servo feed back or give positionl errors. Helical cut racks are usually the preserve of optical systems with lead screws being the norm for machine tooling. This is a generalisation and there are exceptions. There are other refinements to deal with backlash (lost motion in traslating angular pinion to linear movement) and these should not be confused with the helical cutting of the teeth to give smooth transition tooth to tooth.

Hope this helps

Regards

Pat

[blazedforever]

Thanks pat for the info!
I'm planning to build a moving gantry table, just in the process of digitally building the machine via CAD. The table will be rather large in size. As of now my gantry has a travel of 1524 mm and if i can source racks long enough I would like to do double that size for the length. This being said I'm sure ball screw will not work. I suppose I could go with a belt drive system, but I sense more rigidity in a rack design, at least for the y axis. For backlash removal I thought helical gears and spring tension would suffice. To drive the system i have 4 400 oz-in stepper motors which I think I will gear 3:1. You mentioned servo feedback and positional errors due to tooth variability? How much variability are we talking about? I'm considering buying my R&P from HPC Gears International they claim adjacent pitch error (mm):
Mod < 3: 0.006mm
Mod > 3: 0.008mm.
I was going to go with 1.5 mod 20 tooth 20 deg PA. Will i end up having positioning problems? .006 mm is an incredibly small amount.

[wildwestpat]

Hi

The accuracy you quote of 0.006mm is as I read the data sheets you indicated for the tooth to tooh placement. For CNC you need the tolerance in the variation over the total travel you need. This will include the backlash - tooth to tooth variability - tooth placement variability over the length of the traverse. The total is the arithmetic sum of the constituent parts.

Ball screws for the 2 meter horizontal span can sag a bit. There are various makers and they include charts to simplify the design. I assume you have looked at these as there are many examples of machines that are screw driven with similar axis lengths.

Suggest you ask the supplier for the rack details for your application and the price which might be a lot higher than the lengths quoted. You will need to talk to them about transport cost and insurance as 2 meter lengths will bend easily in shipping.

Have you given tensioned toothed belt some consideration. The belt is fasened at both ends and used as a rack with the pinion fixed to the gantry.

Regards

Pat

[wildwestpat]

Hi

Here are the ballscrdata files which may help you decide on the most appropriate drive for your design.

Have fun as this is the best stage - anything is possible until ditched !
Regards

Pat

[blazedforever]

I have decided to go with Item Aluminum extrusions for the gantry frame, they supply various drive systems including belt and chain drives and reversal units to drive the belt/chain along the axis. But what makes a tensioned belt system any better than a rack design when it comes to backlash/tooth variability? is it just because the belt teeth are less rigid and will give a little more? At high speeds wont the elasticity of the belt be more of a problem? I'm guessing the gantry will weigh around 10kg not including the spindle. The inertial force could become quite strong at high speeds.

Item Reversal Unit
Belt tension

Thanks For all the help!

[wildwestpat]

Hi

The Item units look cool but might prove expensive as this is a system that would enable a number of industrial production line machines to be mocked up and quickly altered.

You will need to check on the availability of timing belts that are long enough for your application and that the tooth form is of the low backlash form. The tooth form has to match the pinion and Item have this sorted with their tension unit and 'reversal' unit. Yes they are listed as 50 m being the max length.

In general timing (toothed) belts have a core of steel or kevlar strands on to which are moulded the teeth. The belt is placed under tension to take out the free movement and then to stretch the belt a little. With the 'reversal' unit there will be around 180 degrees of wrap which will place several teeth in full mesh with the pinion. With a wide gantry you will have a similar drive on both sides and a shaft through the 'reversal' units this links the gantry legs and prevents, or reduces, racking to a minimum. The quoted pre load is 1% elongation which makes me think the cores are not a strong as one would like for a precision milling operation.

There are plus and minus features when comparing metal R&P with timing belts.

I general timing belt drives will be less noisy (I am assuming that they are not slobered with thick grease) and more tolerant of dirt than their metal counterparts. The accuracy of the toothed timing belt will probably be inferior to a precision ground metal R&P but the cost will be far less and the setting up of a toothed belt drive is a lot easier than for a metal R&P. I suspect you would have to spring load the rack into the pinion as there would be flex in the frame of the machine that would cause the mesh of the pinion into the metal rack to be less than perfect - spring loading would mean extra friction and wear. It appears that the reversing units are used as a neat way of providing the gantry drive and integration with the alloy section used by Item.

The timing belt is made to an international standard and this will lay down the minimum pitch variation tooth to tooth but the individual source of supply would need to be consulted to find out the tolearance in absolute position of each tooth with respect to a fixed datum over the length of travel - also you would need the length to langth variation and this could be lower if you by as one length. This is a gut feeling as I would go with ball screws!

This link will give you a clue on costing:-

http://www.ondrives.com/commercial-g...gth-belts.html

I don't have any information to hand about the positional accuracy.

Good luck with the design

Pat