OCCRA
Go to Post We need to find the female engineering mentors in FIRST and start having them present/co-present technical conferences at the championships. We need to start showing the girls on the teams that there are female engineering "rock stars" to look up to. - Kims Robot [more]
Home
Go Back   Chief Delphi > Technical > Technical Discussion
CD-Media   CD-Spy  
portal register members calendar search Today's Posts Mark Forums Read FAQ rules

 
Reply
Thread Tools Rate Thread Display Modes
  #1   Spotlight this post!  
Unread 05-11-2018, 09:03 AM
Connor289 Connor289 is offline
Registered User
FRC #3323 (Potential Energy)
Team Role: Mechanical
 
Join Date: Mar 2017
Rookie Year: 2016
Location: New Hampshire
Posts: 13
Connor289 is an unknown quantity at this point
Gear spacing

During the off-season I wanted to expand our capabilities by fabricating a custom gearbox. For simplicity I am just making a two CIM gearbox to start. I figured out the ratio I need and found all the gears off of andymark. The problem I have is figuring out far apart to space the gears for thermal expansion. And also do I use the gear pitch diameter to space them or is there some other measurement I need. None of our mentors have experience with this and I was wondering if any of you guys could help.
Thank you
Reply With Quote
  #2   Spotlight this post!  
Unread 05-11-2018, 09:06 AM
ARaulinaitis ARaulinaitis is offline
Registered User
FRC #4999 (Momentum Robotics)
Team Role: Mentor
 
Join Date: Jan 2018
Rookie Year: 2015
Location: Long Beach, CA
Posts: 146
ARaulinaitis is just really niceARaulinaitis is just really niceARaulinaitis is just really niceARaulinaitis is just really nice
Re: Gear spacing

You will want to use the pitch diameter of the gears to space them.

In terms of thermal expansion, I can't help you there. You can take a look at the equations for thermal expansion and the materials using and see if the expansion is on the order of magnitude of your machining precision. If the thermal expansion you're expecting is +0.0005" on the OD, but your machining tolerance is 0.005", then it might not matter so much.
__________________
Don't limit others to the level of your own creativity.

Spec sheets are your friend.

A problem that you don't want to solve is an opportunity to better yourself.
Reply With Quote
  #3   Spotlight this post!  
Unread 05-11-2018, 09:08 AM
Ty Tremblay's Avatar
Ty Tremblay Ty Tremblay is offline
Robotics Engineer
FRC #0319 (Big Bad Bob)
Team Role: Mentor
 
Join Date: Feb 2006
Rookie Year: 2004
Location: Alton NH
Posts: 1,034
Ty Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond repute
Re: Gear spacing

http://www.wcproducts.net/how-to-gears/
__________________
Reply With Quote
  #4   Spotlight this post!  
Unread 05-11-2018, 09:11 AM
tim-tim's Avatar
tim-tim tim-tim is offline
Simplicity by Design...
AKA: Tim Miedzinski
FRC #0836 (The RoboBees)
Team Role: Mentor
 
Join Date: Feb 2007
Rookie Year: 2004
Location: California
Posts: 644
tim-tim has a reputation beyond reputetim-tim has a reputation beyond reputetim-tim has a reputation beyond reputetim-tim has a reputation beyond reputetim-tim has a reputation beyond reputetim-tim has a reputation beyond reputetim-tim has a reputation beyond reputetim-tim has a reputation beyond reputetim-tim has a reputation beyond reputetim-tim has a reputation beyond reputetim-tim has a reputation beyond repute
Re: Gear spacing

WCP Gear Resource

The rule of thumb is add .003 to center to center for FRC designs; there are caveats...

Use the pitch diameter of the gears. Add the two pitch diameter up, divide by 2 and then add your center to center modification.

Example: 20T and 80T 20dp gears
(1in + 4in)/2 + .003 = 2.503in center to center distance
__________________
The RoboBees

Tim's Shortcuts Anderson Powerpoles and Crimper, Star/Tube Nuts
Reply With Quote
  #5   Spotlight this post!  
Unread 05-11-2018, 10:00 AM
bcampbell's Avatar
bcampbell bcampbell is offline
Registered User
FRC #0294 (Beach Cities Robotics)
Team Role: Mentor
 
Join Date: Apr 2014
Rookie Year: 2003
Location: California
Posts: 68
bcampbell has a spectacular aura aboutbcampbell has a spectacular aura aboutbcampbell has a spectacular aura about
Re: Gear spacing

Here is a little more clarity than just using an online calculator. It is really easy to calculate the center to center distances for gears

Here is the formula for center to center distance.

("#of teeth of gear 1"+"# of teeth of gear 2") / ("pitch Diameter" * 2) + "gear mesh spacing tolerance" = "center to center distance in inches'

For our team we use .003 for gear mesh spacing on 20dp gears and .0015 for 32dp gears.

So for example of a 20dp gear spacing of a 20 tooth and 60 tooth gears would be

(20+60)/(20*2) + .003 = 2.003 inches

An example for the calculations of a 32dp gear spacing for a 12 tooth pinion and a 60 tooth gear would be

(12+60)/(32*2) + .0015 = 1.1265 inches

Hopefully this helps
Reply With Quote
  #6   Spotlight this post!  
Unread 05-11-2018, 10:38 AM
JamesCH95's Avatar
JamesCH95 JamesCH95 is offline
Hardcore Dork
AKA: JCH
FRC #0095 (The Grasshoppers)
Team Role: Engineer
 
Join Date: Dec 2004
Rookie Year: 2001
Location: Enfield, NH
Posts: 2,592
JamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond reputeJamesCH95 has a reputation beyond repute
Re: Gear spacing

Look at coefficients of thermal expansion here: https://www.engineeringtoolbox.com/l...ents-d_95.html

First, if your gearbox and gears are the same material (i.e. aluminum) then CTE won't matter. As the box changes the gears will change the same amount.

If they're different materials, or one is heating up much more than the the other, then let's do a sample calc. I'll assume that your gearbox is aluminum and you're using the gears in tim-tim's example, ctc spacing of 2.503, and the gears are steel with an aluminum gearbox:

Al - 22ppm/C
Steel - 12ppm/C

To see a 0.001in change in ctc spacing, which is about as accurately as many teams can make gearboxes in the first place:

2.503in*10ppm/C*del_T=0.001in

del_T = 0.001in/2.503in/10ppm/C = ~40C. That's the difference between room temperature and 'so hot if you touch the assembly you'll do tissue damage in moments.' In other words, for FRC, you've got bigger problems! I would not worry about CTE.
__________________
Theory is a nice place, I'd like to go there one day, I hear everything works there.

Maturity is knowing you were an idiot, common sense is trying to not be an idiot, wisdom is knowing that you will still be an idiot.
Reply With Quote
  #7   Spotlight this post!  
Unread 05-11-2018, 10:45 AM
Cyberphil's Avatar
Cyberphil Cyberphil is offline
That Guy
FRC #0103 (Cybersonics), FRC 5666 (Purple Lightning)
Team Role: Engineer
 
Join Date: Feb 2008
Rookie Year: 2007
Location: Doylestown, PA
Posts: 1,013
Cyberphil has a reputation beyond reputeCyberphil has a reputation beyond reputeCyberphil has a reputation beyond reputeCyberphil has a reputation beyond reputeCyberphil has a reputation beyond reputeCyberphil has a reputation beyond reputeCyberphil has a reputation beyond reputeCyberphil has a reputation beyond reputeCyberphil has a reputation beyond reputeCyberphil has a reputation beyond reputeCyberphil has a reputation beyond repute
Re: Gear spacing

Quote:
Originally Posted by JamesCH95 View Post
First, if your gearbox and gears are the same material (i.e. aluminum) then CTE won't matter. As the box changes the gears will change the same amount.
I agree with your overall conclusion, that CTE in the FRC scope doesn't really matter, but this statement is assuming the gears and plates are all the same temperature, which in all likelihood will not be the case.
Reply With Quote
  #8   Spotlight this post!  
Unread 05-14-2018, 10:48 AM
Teched3 Teched3 is offline
Hodge1
FRC #0175 (BuzzRobotics)
Team Role: Mentor
 
Join Date: Apr 2006
Rookie Year: 1996
Location: Connecticut
Posts: 325
Teched3 has much to be proud ofTeched3 has much to be proud ofTeched3 has much to be proud ofTeched3 has much to be proud ofTeched3 has much to be proud ofTeched3 has much to be proud ofTeched3 has much to be proud ofTeched3 has much to be proud ofTeched3 has much to be proud of
Smile Re: Gear spacing

For whatever it is worth, years ago I contacted Boston Gear regarding this question. I consider them an expert in this matter. Their answer was to use the sum of the diametral pitch radii to properly space the gears. No need to add more distance between the gears. Do what you want, but I space them this way without problem. When spacing sprockets, I again space the sprockets according to calculations, and ALWAYS provide a method for additional tensioning as needed.
__________________
Sweat the details
Why is there never enough time to do it right, but always enough time to do it again!
Even if you're on the right track, if you sit still, you'll get run over!!!
Reply With Quote
  #9   Spotlight this post!  
Unread 05-14-2018, 10:52 AM
Ty Tremblay's Avatar
Ty Tremblay Ty Tremblay is offline
Robotics Engineer
FRC #0319 (Big Bad Bob)
Team Role: Mentor
 
Join Date: Feb 2006
Rookie Year: 2004
Location: Alton NH
Posts: 1,034
Ty Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond reputeTy Tremblay has a reputation beyond repute
Re: Gear spacing

Quote:
Originally Posted by Teched3 View Post
For whatever it is worth, years ago I contacted Boston Gear regarding this question. I consider them an expert in this matter. Their answer was to use the sum of the diametral pitch radii to properly space the gears. No need to add more distance between the gears. Do what you want, but I space them this way without problem. When spacing sprockets, I again space the sprockets according to calculations, and ALWAYS provide a method for additional tensioning as needed.
This is correct, except gear profiles were designed to allow for a little wear in over the life of the gears, and get a little more efficient with a little run-in. FRC robots aren't used long enough for this designed run-in to occur, so you can achieve a similar effect by moving the gears a little further apart. This way you can start with a slightly more efficient (quieter / smoother) geartrain at the sacrifice of additional backlash.
__________________
Reply With Quote
  #10   Spotlight this post!  
Unread 05-14-2018, 12:29 PM
Peter Johnson Peter Johnson is online now
Volunteer WPILib Developer, CSA
no team (WPILib)
 
Join Date: Jan 2010
Rookie Year: 2008
Location: Redondo Beach, CA
Posts: 532
Peter Johnson has a reputation beyond reputePeter Johnson has a reputation beyond reputePeter Johnson has a reputation beyond reputePeter Johnson has a reputation beyond reputePeter Johnson has a reputation beyond reputePeter Johnson has a reputation beyond reputePeter Johnson has a reputation beyond reputePeter Johnson has a reputation beyond reputePeter Johnson has a reputation beyond reputePeter Johnson has a reputation beyond reputePeter Johnson has a reputation beyond repute
Re: Gear spacing

Quote:
Originally Posted by Ty Tremblay View Post
This is correct, except gear profiles were designed to allow for a little wear in over the life of the gears, and get a little more efficient with a little run-in. FRC robots aren't used long enough for this designed run-in to occur, so you can achieve a similar effect by moving the gears a little further apart. This way you can start with a slightly more efficient (quieter / smoother) geartrain at the sacrifice of additional backlash.
The other reason to add a little extra space is machining tolerances. A lot of teams struggle to hold tolerances within 1-2 thou unless they are using a CNC. And even with a CNC machining the plates, it's easy to get plate misalignment that's more than that (e.g. two plates that are clocked a little bit because they don't have good alignment) unless you're explicitly designing in tight-tolerance alignment features. Designing in a little extra space is an easy way to ensure your gear teeth don't bottom out and jam.
__________________
WPILib Developer, 2014-Present:
  • Author of cscore - WPILib CameraServer for 2017+
  • Author of ntcore - WPILib NetworkTables for 2016+
Creator of RobotPy - Python for FRC
Mentor, Team 294, 2008-2018 (World Champions FRC 2010 & FTC 2008)
Reply With Quote
  #11   Spotlight this post!  
Unread 05-14-2018, 01:20 PM
rcmolloy's Avatar
rcmolloy rcmolloy is offline
Lead Technical Mentor
AKA: Robert Cory Molloy
FRC #2659 (RoboWarriors)
Team Role: Mentor
 
Join Date: Jan 2009
Rookie Year: 2009
Location: Los Angeles, California
Posts: 478
rcmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond repute
Re: Gear spacing

Quote:
Originally Posted by Teched3 View Post
For whatever it is worth, years ago I contacted Boston Gear regarding this question. I consider them an expert in this matter. Their answer was to use the sum of the diametral pitch radii to properly space the gears. No need to add more distance between the gears. Do what you want, but I space them this way without problem. When spacing sprockets, I again space the sprockets according to calculations, and ALWAYS provide a method for additional tensioning as needed.
Sum of the diametral pitch radii?

Could you go into more detail or provide an example?
__________________
FRC 1647: Iron Devils - 2009 - 2011
FRC 973: Greybots - 2011 - 20XX (Honorary Mentor)
FRC 2659: RoboWarriors - 2017 -2018

"Life's about making mistakes but it's also about trying to be great."
Reply With Quote
  #12   Spotlight this post!  
Unread 05-14-2018, 01:29 PM
AriMB's Avatar
AriMB AriMB is offline
The Philadelphian emigrant
AKA: Ari Meles-Braverman
FRC #5987 (Galaxia)
Team Role: Mentor
 
Join Date: Mar 2015
Rookie Year: 2012
Location: Haifa, Israel
Posts: 1,717
AriMB has a reputation beyond reputeAriMB has a reputation beyond reputeAriMB has a reputation beyond reputeAriMB has a reputation beyond reputeAriMB has a reputation beyond reputeAriMB has a reputation beyond reputeAriMB has a reputation beyond reputeAriMB has a reputation beyond reputeAriMB has a reputation beyond reputeAriMB has a reputation beyond reputeAriMB has a reputation beyond repute
Re: Gear spacing

Quote:
Originally Posted by rcmolloy View Post
Sum of the diametral pitch radii?

Could you go into more detail or provide an example?
Pitch diameter for a given gear is (# of teeth)/(diametrical pitch). Diametrical pitch radius is half of that (radius vs. diameter), or (1/2)*(# of teeth)/(diametrical pitch)

So the exact C-C distance for a given gear pair would be d=t1/(2*dp1)+t2/(2*dp2). Since the diametrical pitch of a gear pair must be the same for the two gears, you get the formula d=(t1+t2)/(2*dp)

For example, if you had a gear pair consisting of a 20t and a 50t gear (20dp), the exact C-C distance would be (20+50)/(2*20)=1.75". It's standard in FRC to add 0.003" to that for reasons listed above, so your final distance would be 1.753".
__________________
Studying MechE at the Technion - Israel Institute of Technology
2017-present: FIRST Israel CSA/FTAA
2017-present: FRC 5987 Technical Mentor 18isr2 18isr4 18isrcmp 18carv
2012-2016: FRC 423 Member 15njtab 15padre 16paphi

Last edited by AriMB : 05-14-2018 at 01:33 PM. Reason: added example
Reply With Quote
  #13   Spotlight this post!  
Unread 05-14-2018, 01:55 PM
rcmolloy's Avatar
rcmolloy rcmolloy is offline
Lead Technical Mentor
AKA: Robert Cory Molloy
FRC #2659 (RoboWarriors)
Team Role: Mentor
 
Join Date: Jan 2009
Rookie Year: 2009
Location: Los Angeles, California
Posts: 478
rcmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond reputercmolloy has a reputation beyond repute
Re: Gear spacing

Quote:
Originally Posted by AriMB View Post
Pitch diameter for a given gear is (# of teeth)/(diametrical pitch). Diametrical pitch radius is half of that (radius vs. diameter), or (1/2)*(# of teeth)/(diametrical pitch)

So the exact C-C distance for a given gear pair would be d=t1/(2*dp1)+t2/(2*dp2). Since the diametrical pitch of a gear pair must be the same for the two gears, you get the formula d=(t1+t2)/(2*dp)

For example, if you had a gear pair consisting of a 20t and a 50t gear (20dp), the exact C-C distance would be (20+50)/(2*20)=1.75". It's standard in FRC to add 0.003" to that for reasons listed above, so your final distance would be 1.753".
This was explained in a separate post above and a calculation that I've used extensively.

The phrasing "sum of diametral pitch radii" was a bit confusing. I can understand where Teched3 was coming from when he contacted Boston Gear and assume that his "0.003 spacing" was not something they recommended, only the exact c-c.

I've honestly never heard of the radius of the pitch diameter referred to as the diametral pitch radius. I'll add that one in my jargon notebook.
__________________
FRC 1647: Iron Devils - 2009 - 2011
FRC 973: Greybots - 2011 - 20XX (Honorary Mentor)
FRC 2659: RoboWarriors - 2017 -2018

"Life's about making mistakes but it's also about trying to be great."

Last edited by rcmolloy : 05-14-2018 at 01:58 PM.
Reply With Quote
Reply


Thread Tools
Display Modes Rate This Thread
Rate This Thread:

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Forum Jump


All times are GMT -5. The time now is 05:39 AM.

The Chief Delphi Forums are sponsored by Innovation First International, Inc.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2018, Jelsoft Enterprises Ltd.
Copyright © Chief Delphi