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  #1  
Old Tue 04 March 2008, 19:00
eloid
Just call me:
 
Backlash in the rack & pinion system

For rack & pinion applications requiring a more precise positioning and repeatability, the backlash can be eliminated by using two pinions - one to drive the axis and one to "preload" the axis to remove the backlash.

The simplest zero-backlash rack & pinion drive utilizes . A split-pinion consists of two pinion halves and an axial spring pack. The pinions halves mesh with opposite tooth flanks on the same rack, eliminating the backlash. The axial spring pack is used to preload the rack & pinions to insure that no backlash develops during the operating cycle.

Has anyone try this? Does anyone know where to get a wide module 1 rack
thats 30 mm wide with going to module 2. Im guessing another option is to put racks side buy side and adjust the pinion to gain the same thing as the attached picture.

the picture shows helical rack but does need to be helical for this to work, Just to confirm the largest rack size should be module 1 or 20 pitch imperial for
rack pinion set up.

check out http://www.atlantadrives.com/zerobacklash.htm
Attached Images
File Type: jpg splitpinion.jpg (17.3 KB, 611 views)

Last edited by eloid; Tue 04 March 2008 at 19:11..
  #2  
Old Tue 04 March 2008, 19:18
eloid
Just call me:
 
do some reasearch in this area, i was ask by sales person the following questions regarding the split pinion design..


Selecting the right split-pinion depends on many things, including load requirements, the duty cycle, and design constraints. Typically, the peak cycle torque can be used to make a preliminary selection.

Does anyone know how to calculate or know aprox best case value of the duty cycle and cycle torque, im guess there a general rule of thumb that
can determined.
  #3  
Old Tue 04 March 2008, 22:20
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
The split pinion gives you exactly the same effect as spring-loading the motor mount like we have it on the MechMate. At a much lower price.

Eloid, are you planning to build a MechMate, or are you just using this forum to gather a lot of information?
  #4  
Old Wed 05 March 2008, 05:45
eloid
Just call me:
 
rack and pinion

"The split pinion gives you exactly the same effect as spring-loading the motor mount like we have it on the MechMate. At a much lower price."

It true the spring is a low cost solution, but relalistical it will not pre load the rack nor take out "ALL" side to side backlash of the pinion in both direction. I dont think the spring will totally improve the side to side play / backlash of the rack/ pinion especialy as the pinion wears. Looking to design improve the setup for "my needs", which will be for aluminun machining and wood working.

as you noted in post #9

Backlash only develops when the pinion gears wear out.

Last edited by eloid; Wed 05 March 2008 at 05:48..
  #5  
Old Wed 05 March 2008, 05:53
smreish
Just call me: Sean - #5, 28, 58 and others
 
Orlando, Florida
United States of America
I found that backlash was one of my minor concerns when machining aluminum on a CNC. Either a Shopbot, Multicam, HAAS or Mechmate. Steps of resolution and acceleration plays just as important role.

Eloid, I ask the question to you - since I have already done this a few times on my own. What "design" elements would you change to keep the fail safe operation of the spring loaded motor plate in place if you went to a different style of pinion.

The CNC will fail while cutting a part. It happens often due to user error or material failure...not because the machine is faulty.

Do you have a back up plan to replace the rack and pinion when it doesn't disengage and grind up a pile of teeth on the floor? Those split pinions, which I have used before, are okay, but are really pricey and I find they start to prematurely wear a groove on centerline of the rack over time.

Backlash is important, but there is a number of other design factors that are paramount to backlash in my book.

Just my 2 cents.
Sean
  #6  
Old Wed 05 March 2008, 06:52
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Quote:
Originally Posted by eloid View Post
It true the spring is a low cost solution, but relalistical it will not pre load the rack nor take out "ALL" side to side backlash of the pinion in both direction.
I am getting tired of this . . .

Would you please be so kind as to make me a sketch of where you see the backlash between the pinion and the rack?
  #7  
Old Wed 05 March 2008, 10:05
DMS
Just call me: Sharma #9 India
 
Rajasthan
India
Gerald,
I received my racks and pinions today, there is no backlash at all, I am very pleased at it.
Thanks for your perfect rack and pinion design.
  #8  
Old Wed 05 March 2008, 10:45
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
I am really tired of reading on the typical CNC forums that rack&pinion is a bad idea because of backlash. Everybody just knows you have use ballscrews if you want to have a "real" CNC machine. But, because this is a big table, they cannot find ballscrews for this length. Reluctantly they accept the use of rack&pinion, but immediately start with designs and theories to remove the backlash that they have been told they are going to get. They can't believe that a swinging motor on a spring cures it completely.

The idea is not mine. I first saw it my ship's radar system in 1979 when I was conscripted to the navy. Then again on a pair of plasma cutting tables about 80 ft long in about '92 and then on the ShopBot and EZ Router tables. (ShopBot have since moved away from it). It's a very low-tech approach, but it works.
  #9  
Old Wed 05 March 2008, 10:49
Marc Shlaes
Just call me: Marc
 
Cleveland, OH
United States of America
Send a message via Skype™ to Marc Shlaes
I have had a lot of people influence my career. Several have said: "Don't invent problems to be solved."

I believe that enough "real" problems exist to go around.
  #10  
Old Wed 05 March 2008, 10:59
Greg J
Just call me: Greg #13
 
Hagerman, New Mexico
United States of America
Totally agree with Marc and Gerald,

I'll change the design when it becomes an issue. I did change the base table for personal reasons, but only that.

When someone (Gerald) has numerous machines operating over a long period of time with no problems, then I think its a non-issue (the rack and pinion debate).
  #11  
Old Wed 05 March 2008, 11:15
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
There are probably about 4000 ShopBots with that system - search their active forum for complaints . . . . .
  #12  
Old Wed 05 March 2008, 18:23
eloid
Just call me:
 
4 simple questions, to answer?

I think everyone should, understand.. not everyone access to the same parts
resources nor equipment to create a carboncopy mechmate cnc.. everyone has there own unquie design/ need and most importanly budget and needs.

I think most of you did not read all of my post, and really ran away with it ....to some wired unknown and unwanted direction?. Please look and read again.. This is was I was asking if it was unclear? I have removed the background info regarding the dual pinion set up. Not to cloud the question.

1)Has anyone try this?
2)Does anyone know where to get a wide module 1 rack
thats 30 mm wide without going to module 2.
3)Im guessing another option is to put racks side buy side and adjust the pinion to gain the same thing as the attached picture.
4)Just to confirm the largest rack size should be module 1 or 20 pitch imperial for rack pinion set up.
  #13  
Old Wed 05 March 2008, 18:27
eloid
Just call me:
 
For smreish

Eloid, I ask the question to you - since I have already done this a few times on my own. What "design" elements would you change to keep the fail safe operation of the spring loaded motor plate in place if you went to a different style of pinion.

No where did i say nor suggest i was removing, changing altering the spring load!, It is fair to said you should not assume...... do you really think someone would preload it that much and still, have the cariage moveable by hand with the gearbox removed? The Most Noticable Backlash develops when the pinion gears wear out, this is a way to keep under control as it W E A R S! NO spring will "totally remove" / help you in that case. key word "totally"

Do you have a back up plan to replace the rack and pinion when it doesn't disengage and grind up a pile of teeth on the floor?

Yes I do... my back ground is electronics and software eng......

Those split pinions, which I have used before, are okay, but are really pricey and I find they start to prematurely wear a groove on centerline of the rack over time.

The pinion im going to use are no different and any other compent of my cnc..
standard off the shelf compents all others .... DIY (again dont assume) with some modification to add a preload disk spring, if your reread my posst again,.... "Im guessing another option is to put racks side buy side and adjust the pinion to gain the same thing as the attached picture". to resolve your prematurely wear a centerline groove. ( cost one pinion some extra rack and a beveville disc spring... my time to make)

This is not a complant Gerald D..."There are probably about 4000 ShopBots with that system - search their active forum for complaints . . . . ."
I am making changes for "my design" for "my needs and wants... for "my investment" in the hobby and equipment "And everyone should re-read paragraph one again of this post...

Last edited by eloid; Wed 05 March 2008 at 18:30..
  #14  
Old Wed 05 March 2008, 22:40
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Eloid, the purpose of this forum is to help people understand the MechMate plans, and to fill in the gaps that the plans do not address. I do not want this forum to become a second CNCZone.

The MechMate plans are quite clear on the rack & pinion, therefore your questions are filling this forum with useless words and wasting my time and energy.

I do not suggest that the plans are perfect, nor have I claimed that a Mechmate can cut anything and everything to perfect finishes in superfast time. If somebody has built the machine and finds a fault in the design, I will move heaven and earth to fix it. But I am not going to waste time chasing somebody else's theoretical standards.
  #15  
Old Thu 06 March 2008, 04:54
eloid
Just call me:
 
Im not as you to do so just using the knowledge base
to help me ? make changes to my needs!
  #16  
Old Thu 06 March 2008, 06:29
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
CNCZone
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