Go to Post I almost forgot the biggest advantage of all! Three times as many people get to hear me and Karthik sing at 2 am! - Derek Bessette [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 06-08-2018, 07:31 PM
backflippingcat backflippingcat is offline
Registered User
AKA: Sidney
FRC #6898 (The Undergraduate School of Carrots)
Team Role: Leadership
 
Join Date: Jun 2017
Rookie Year: 2013
Location: north texas
Posts: 44
backflippingcat will become famous soon enough
Pivots?

What are some light/effective/elegant/long-term solutions for pivot points? For example, the pivots or points of rotation on teams' cube intakes this year (especially those with a left/right wheeled intake with some sort of spring/pneumatic system to grip the cube) -- any recommendations for how to execute those well? Thanks in advance for the help!
Reply With Quote
  #2   Spotlight this post!  
Unread 06-08-2018, 08:13 PM
rklopp's Avatar
rklopp rklopp is offline
Registered User
FRC #0100 (Wildhats)
Team Role: Mentor
 
Join Date: Jan 2017
Rookie Year: 2016
Location: Woodside, CA
Posts: 16
rklopp is an unknown quantity at this point
T100 did our pivots with typical 1/2 hex x 1-1/8 flange bearings on hex shaft, because the pivots were both pivots and shafts for timing belt pulleys driving the intake wheels.


Sent from my iPhone using Tapatalk
Reply With Quote
  #3   Spotlight this post!  
Unread 06-08-2018, 08:21 PM
pkrishna3082's Avatar
pkrishna3082 pkrishna3082 is online now
love/hate relationship with swerve
AKA: Parthiv Krishna
FRC #3082 (Chicken Bot Pie) and FTC #13471 Mentor (Minnetonka)
Team Role: Leadership
 
Join Date: Mar 2017
Rookie Year: 2016
Location: Minnetonka, MN, USA
Posts: 785
pkrishna3082 has a reputation beyond reputepkrishna3082 has a reputation beyond reputepkrishna3082 has a reputation beyond reputepkrishna3082 has a reputation beyond reputepkrishna3082 has a reputation beyond reputepkrishna3082 has a reputation beyond reputepkrishna3082 has a reputation beyond reputepkrishna3082 has a reputation beyond reputepkrishna3082 has a reputation beyond reputepkrishna3082 has a reputation beyond reputepkrishna3082 has a reputation beyond repute
Re: Pivots?

We originally used hex shafts in standard 1.125" OD bearings, since we were planning to use them to drive the intake wheels as well. We ended up putting motors on the arms of the intake, so we switched over to 1/2" bolts inside bronze bushings, since the bushings seemed to support the sides a bit better (they had less play compared to the ball bearings). Plus, it was only a ~1/4 rotation movement, so the bearings weren't really necessary.
__________________

My posts constitute my opinions, and mine alone, and are not representative of the views held by my team.
2010-2011: FLL 11083, 13873 (Minnetonka Silver) - Programmer
2016-: FRC 3082 (Chicken Bot Pie) - Engineering Captain, fmr. Programming Lead, 2017&2018 Dean's List Finalist
2017-: FTC 13471 (Minnetonka) - Founding Mentor
Reply With Quote
  #4   Spotlight this post!  
Unread 06-08-2018, 08:22 PM
AdamHeard's Avatar
AdamHeard AdamHeard is offline
Lead Mentor
FRC #0973 (Greybots)
Team Role: Mentor
 
Join Date: Oct 2004
Rookie Year: 2004
Location: Atascadero
Posts: 5,988
AdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond reputeAdamHeard has a reputation beyond repute
Send a message via AIM to AdamHeard
Re: Pivots?

Bearings or bushings for high load, speed and/or duty cycle.

You can get away with bolts as pivots for lighter duty things or for systems that adjust only a few degrees (like a 2013 shooter deck being asjusted by pneumatic cylinders 3*)
__________________
Get your cascade elevators and PowerCube claws!
2x1 and 2x2 1/16" wall tubing for sale!

Need help? Check out 973 Remote Assistance and Mentorship Program.
Like our Robots? Check out the CAD files of MOST of our robots!

2017 "World Champions"
2014 Galileo Champions
2011 World Champions
Reply With Quote
  #5   Spotlight this post!  
Unread 06-08-2018, 08:33 PM
bstew's Avatar
bstew bstew is offline
Registered User
FRC #3928 (Team Neutrino)
Team Role: CAD
 
Join Date: Mar 2015
Rookie Year: 2015
Location: Ames, IA
Posts: 109
bstew is a splendid one to beholdbstew is a splendid one to beholdbstew is a splendid one to beholdbstew is a splendid one to beholdbstew is a splendid one to beholdbstew is a splendid one to beholdbstew is a splendid one to beholdbstew is a splendid one to behold
Re: Pivots?

3928 likes to use Oil-Embedded Flanged Sleeve Bearings. Just press them in and put your pivot on a round shaft or shoulder bolt.
__________________


Reply With Quote
  #6   Spotlight this post!  
Unread 06-08-2018, 09:55 PM
Type's Avatar
Type Type is offline
Registered User
AKA: AJ
FRC #3452 (GreengineerZ)
Team Role: Mechanical
 
Join Date: Jan 2016
Rookie Year: 2016
Location: Michigan
Posts: 419
Type has much to be proud ofType has much to be proud ofType has much to be proud ofType has much to be proud ofType has much to be proud ofType has much to be proud ofType has much to be proud ofType has much to be proud ofType has much to be proud of
Re: Pivots?

We used 1/2" round for our intake pivot side to side this year, one being just a 1/2" hole and the other being where the bearings are, then 5/16" shoulder bolts for the rotation point at the start of the match to drop the intake.
__________________
3452- Lead Builder/ Pit Boss





*My posts do not reflect the opinion of my team*
Reply With Quote
  #7   Spotlight this post!  
Unread 06-09-2018, 12:02 AM
johncooper johncooper is offline
Registered User
FRC #6072 (Triton Tech Robotics)
Team Role: Mentor
 
Join Date: Jun 2017
Rookie Year: 2016
Location: Newport Beach
Posts: 24
johncooper is an unknown quantity at this point
Re: Pivots?

Quote:
Originally Posted by bstew View Post
3928 likes to use Oil-Embedded Flanged Sleeve Bearings. Just press them in and put your pivot on a round shaft or shoulder bolt.
This. Compared to a bearing, less space, less cost and easier installation. Worked flawlessly.
Reply With Quote
  #8   Spotlight this post!  
Unread 06-09-2018, 12:14 AM
MrForbes's Avatar
MrForbes MrForbes is online now
Registered User
AKA: Jim
FRC #1726 (N.E.R.D.S.)
Team Role: Mentor
 
Join Date: Feb 2006
Rookie Year: 2006
Location: Sierra Vista AZ
Posts: 6,401
MrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond repute
Re: Pivots?

Quote:
Originally Posted by AdamHeard View Post
You can get away with bolts as pivots for lighter duty things or for systems that adjust only a few degrees (like a 2013 shooter deck being asjusted by pneumatic cylinders 3*)
We've gotten away with using bolts for pivots for a whole bunch of things...

one trick is to use a long bolt, with two nuts, one nut on either side of the part that the pivot is attached to. Then tighten the nuts against each other, and the part that pivots can be on the bolt, retained loosely by the head of the bolt. hard to describe, and even harder for students to figure out which things to tighten...but it works pretty well, and don't cost much, or take much machining. Just drill a few holes.

Reply With Quote
  #9   Spotlight this post!  
Unread 06-09-2018, 03:51 AM
mman1506 mman1506 is offline
Reversible bumpers are my trigger
AKA: Marcus Quintilian
FRC #5406 (Celt-X)
Team Role: Mechanical
 
Join Date: Mar 2012
Rookie Year: 2012
Location: Toronto
Posts: 1,359
mman1506 has a reputation beyond reputemman1506 has a reputation beyond reputemman1506 has a reputation beyond reputemman1506 has a reputation beyond reputemman1506 has a reputation beyond reputemman1506 has a reputation beyond reputemman1506 has a reputation beyond reputemman1506 has a reputation beyond reputemman1506 has a reputation beyond reputemman1506 has a reputation beyond reputemman1506 has a reputation beyond repute
Re: Pivots?

Quote:
Originally Posted by MrForbes View Post
We've gotten away with using bolts for pivots for a whole bunch of things...

one trick is to use a long bolt, with two nuts, one nut on either side of the part that the pivot is attached to. Then tighten the nuts against each other, and the part that pivots can be on the bolt, retained loosely by the head of the bolt. hard to describe, and even harder for students to figure out which things to tighten...but it works pretty well, and don't cost much, or take much machining. Just drill a few holes.

Quote:
Originally Posted by OP
What are some light/effective/elegant/long-term solutions for pivot points?
That is none of those.
__________________
2012-2015: 865 Warp7 Team Captain
2016/17: Free Agent Mentor
2018: 5406 Celt-X
Reply With Quote
  #10   Spotlight this post!  
Unread 06-09-2018, 08:18 AM
GeeTwo's Avatar
GeeTwo GeeTwo is offline
Mentor
AKA: Gus Michel II
FRC #3946 (Tiger Robotics)
Team Role: Mentor
 
Join Date: Jan 2014
Rookie Year: 2013
Location: Slidell, LA
Posts: 5,543
GeeTwo has a reputation beyond reputeGeeTwo has a reputation beyond reputeGeeTwo has a reputation beyond reputeGeeTwo has a reputation beyond reputeGeeTwo has a reputation beyond reputeGeeTwo has a reputation beyond reputeGeeTwo has a reputation beyond reputeGeeTwo has a reputation beyond reputeGeeTwo has a reputation beyond reputeGeeTwo has a reputation beyond reputeGeeTwo has a reputation beyond repute
Re: Pivots?

Quote:
Originally Posted by MrForbes View Post
We've gotten away with using bolts for pivots for a whole bunch of things...
We've done this - a lot lighter and simpler and more robust than idle hex shafts in hex bearings! For lighter duty pivots, including our off-season robot's arm to raise a cube about 20" and to 45 degrees so we can launch it into a switch, we use hinges - sometimes riveted, sometimes bolted depending on the forces. For 2017 STEAMworks, we had spring hinges on two doors and a popup gear intake ramp.
__________________

If you can't find time to do it right, how are you going to find time to do it over?
If you don't pass it on, it never happened.
Robots are great, but inspiration is the reason we're here.
Friends don't let friends use master links.
Reply With Quote
  #11   Spotlight this post!  
Unread 06-09-2018, 09:31 AM
MrForbes's Avatar
MrForbes MrForbes is online now
Registered User
AKA: Jim
FRC #1726 (N.E.R.D.S.)
Team Role: Mentor
 
Join Date: Feb 2006
Rookie Year: 2006
Location: Sierra Vista AZ
Posts: 6,401
MrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond repute
Re: Pivots?

Quote:
Originally Posted by mman1506 View Post
That is none of those.
I guess you have a different conception of these principles, than I do.

That's ok.
Reply With Quote
  #12   Spotlight this post!  
Unread 06-09-2018, 11:53 AM
RoboChair's Avatar
RoboChair RoboChair is offline
He who fixes with hammers #tsimfd
AKA: Devin Castellucci
FRC #1678 (Citrus Circuits and 5458 Digital Minds)
Team Role: Mentor
 
Join Date: Jan 2007
Rookie Year: 2005
Location: Davis, CA
Posts: 1,095
RoboChair has a reputation beyond reputeRoboChair has a reputation beyond reputeRoboChair has a reputation beyond reputeRoboChair has a reputation beyond reputeRoboChair has a reputation beyond reputeRoboChair has a reputation beyond reputeRoboChair has a reputation beyond reputeRoboChair has a reputation beyond reputeRoboChair has a reputation beyond reputeRoboChair has a reputation beyond reputeRoboChair has a reputation beyond repute
Re: Pivots?

Quote:
Originally Posted by mman1506 View Post
That is none of those.
I'll give you that it isn't elegant, but it is certainly light, effective, and capable of lasting more than a season.
__________________

12 Years and counting! Over a third of my life has been spent with FRC.
Reply With Quote
  #13   Spotlight this post!  
Unread 06-09-2018, 12:27 PM
Inconel-oh-el Inconel-oh-el is offline
Registered User
FRC #3936 (RoboBlitz)
Team Role: Mentor
 
Join Date: May 2018
Rookie Year: 2013
Location: Philadelphia
Posts: 11
Inconel-oh-el is on a distinguished road
Re: Pivots?

Quote:
Originally Posted by MrForbes View Post
I guess you have a different conception of these principles, than I do.

That's ok.
Solutions like these honestly work acceptably if working under a deadline or at a competition, but should never be designed into the robot from the start if you try to follow best practices. Standard bolts are soft and make for terrible shafts/bearing surfaces, the threads of them make for even worse. Higher contact pressures at the crests of the threads will quickly wear away the bearing material, or vice-versa, especially if just drilled through alu. frame members. Even for stationary loads, avoid any shear forces on the threads as excess stress risers in the root will lead to decreased fatigue life/increased risk of failure. Ideally, only the shank (unthreaded portion) of the bolt is loaded in double shear (supported on both ends) and the threads/lock-nut simply retains and tensions the fastener.



More to the OP's question: For rotating joints, Shoulder Bolts work well as they are made from harder steel and ground for precise bearing surfaces and fits They also can greatly simplify an assembly over steel shaft and collars. The stationary portion of your mechanism should support the shank at both ends. The bolt does not rotate in this arrangement- all sliding interactions should happen between the ground shank of the bolt and the inner surface of the bearing. Press fit the bearing into the rotating portion of your mechanism. Oil-Embedded Flanged Sleeve Bushings work well for low speed mechanisms or incomplete rotations. Needle-Roller Bearings work well for higher speeds or when low-friction is required of the joint. Really only use them on hardened steel shafts, they will chew through aluminum pretty easily, but you can get shaft liners for larger IDs. Keep in mind any axial forces in the joint as well, as needle roller bearings will not support these. I like Nylon Thrust Washers for lighter loads as you can also stack or custom turn them to remove axial play if need be. There are other types of thrust bearings as well though, and put one on each side against the stationary faces to keep the joint from seizing when you tighten the lock-nut. Good surface finish helps. Also pay attention to any bending moments applied to the joint. Use two bearings on both sides of the rotating mechanism, and try to minimize the space in between the bearing and the inner faces of the joint, to keep the shear force on the bolt closer to ideal shear.

This is engineering "best practice" for single DOF joints with high strength, and low backlash requirements. For race teams, a common use case is the suspension rocker arms. They redirect the motion of the push/pull arms into the shocks/dampers and experience extremely high and repeated load, so fatigue is a large factor. The joints must have no play as that will translate into suspension compliance, giving the driver a poor and unpredictable feel for the car. They also must be low friction as to minimize the joints damping effect. The actual dampers should handle all damping, as that can be tuned when setting up the car, and the telemetry will more closely match the predicted model. Stiff joints will also wear quicker, bad all around, so from a maintainability standpoint these components can be replaced instead of the whole frame.

I hope this helps you OP, and I can answer some questions in regards to FRC design/manufacturability as we employed this method on the wrist joint of our intake this year, and several other places.



(full disclosure: after a revision to the carriage before the second competition, I neglected to order a new shoulder bolt for the rear pivot of the pneumatic piston that lifted the intake. We used a bolt through to Detroit. The threads bore directly on the bronze bushing we had, and the load was applied FAR from ideal shear. As expected, it quickly wore into the bushing, despite us actuating the piston a few times a match. I can't say I'm proud, but that was far from the example of "worst engineering practice". Your robot won't fall apart from a joint like that. Race cars would though)
Reply With Quote
  #14   Spotlight this post!  
Unread 06-09-2018, 12:36 PM
MrForbes's Avatar
MrForbes MrForbes is online now
Registered User
AKA: Jim
FRC #1726 (N.E.R.D.S.)
Team Role: Mentor
 
Join Date: Feb 2006
Rookie Year: 2006
Location: Sierra Vista AZ
Posts: 6,401
MrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond reputeMrForbes has a reputation beyond repute
Re: Pivots?

I thought we were designing robots, for a competition season. I don't use this design on my race car!

My idea of "elegant" includes figuring out what the requirements are, and designing to meet those requirements with the least cost and effort. Simplicity is good.

I guess next year's design will include the requirement to get into deep space, so we'll have to step up our game. Or maybe it will still just be a high school robotics competition?

edit: oops! I forgot...I actually did use this design on my race car, to hold the hood prop. It works great.


Last edited by MrForbes : 06-09-2018 at 12:45 PM.
Reply With Quote
  #15   Spotlight this post!  
Unread 06-09-2018, 01:18 PM
Inconel-oh-el Inconel-oh-el is offline
Registered User
FRC #3936 (RoboBlitz)
Team Role: Mentor
 
Join Date: May 2018
Rookie Year: 2013
Location: Philadelphia
Posts: 11
Inconel-oh-el is on a distinguished road
Re: Pivots?

Quote:
Originally Posted by MrForbes View Post
I thought we were designing robots, for a competition season. I don't use this design on my race car!

My idea of "elegant" includes figuring out what the requirements are, and designing to meet those requirements with the least cost and effort. Simplicity is good.

I guess next year's design will include the requirement to get into deep space, so we'll have to step up our game. Or maybe it will still just be a high school robotics competition?


Simplicity is a great thing, I completely agree. On critical and non-critical mechanisms, I never want to have to worry about them failing or replacing parts on them or having to fiddle with bolt tension or ten shaft collars spaced out across the shaft- I want them to just work! Well designed joints are simple in this regard. Often lots of thought goes towards simplifying the manufacturing, and less so the reliability or maintainability.

The second half to my post was addressed at the OP who asked for examples, I know that you know how to build race cars . Its just an example use case that others can add to their repertoire, and hopefully I explained the design requirements and intent behind it well enough in my post that they would know when to use methods like this.... For me, FSAE was a natural step from FRC. Judges there will tear cars apart in design reviews if they see stuff like this. So I want my students to put that much thought into even "simple" systems like a high-school robotics competition. And it won't be next year, but eventually they will be required to get to deep space!

Quote:
Originally Posted by MrForbes View Post
edit: oops! I forgot...I actually did use this design on my race car, to hold the hood prop. It works great.
Disgusting. I bet it's in single shear as well, you monster. That's going to break. GTB!
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 09:55 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