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| Request for help with shaft deflection problem |  | ||
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Posted by: john2003 Ū 10/03/2005, 19:00:20 Author Profile Mail author Edit |
Hello everyone, I have a 2mm OD hardened steel dowel pin used as a bushing shaft, that is 0.505 inches long. Each end of the dowel is supported by a small Igus plastic bushing, Part # GSM-0203-03, which are each 3mm long. The bushings are located flush with the ends of the dowel. The clearance between the shaft OD and the bushing ID will be from .0006 Minimum to 0.003 Maximum. The distance between the inside edges of the two bushings will be 0.269 inch. In the center of this .269 span, a 3/16 OD X 3/16 long steel tube is pressed onto the 2mm OD dowel pin. The tube acts as a small roller or cam-follower. When the roller rotates, the 2mm OD dowel rotates with the roller, since the roller is pressed onto the dowel and is basically like one piece. I have a .031 thick thrust washer located on each side of the roller, within the 0.269 wide span. If a shaft is simply supported at each end, and you put a load in the middle of the supports, not only does the shaft deflect down at the center, but the ends of the shaft will tend to deflect & curl up as well. I can calculate the shaft deflections if the 3/16 OD X 3/16 long tube were not pressed onto the shaft, but after the tube is pressed onto the shaft, its as if the center of the shaft has a 3/16 OD and the two ends have a 2mm OD. The 3/16 OD tubing stiffens everything up. Can anyone please tell me how to calculate the deflection of the shaft after the 3/16 OD X 3/16 long steel tubing is pressed onto the dowel ? I will have a 200 pound load on the roller, the cam is 3/16 wide just like the roller, so I suppose you would consider this to be a distributed load. I am not concerned with deflections inside of the .269 span because I think they will be very small, probably less than .0005. I am concerned with how far the very ends of the dowel will curl up or deflect, since this could produce misalignment and /or binding of the dowel / shaft in the ID of the bushing. I would appreciate any feedback anyone can offer. Thanks for your help.
Modified by Administrator at Mon, Oct 03, 2005, 19:04:01 |
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| Re: Request for help with shaft deflection problem -- john2003 | Post Reply | Top of thread | Forum |
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Posted by: john2003 Ū 10/10/2005, 13:42:09 Author Profile Mail author Edit |
Hi everyone, I think I have found a way to greatly reduce the stresses on the 2mm OD shaft. The only question is whether the 3/16" OD roller being pressed onto the center of the 2mm OD dowel, will basically act like a stepped shaft made from one solid piece ? Since the stresses were so high, I decided to take a closer look at this problem, rather than just relying on a physical test. With variances in steel, I could have a few that would test OK, but others that would not. Also, if I tested one and it seemed OK, I am afraid it could yield a little more with each use, and then cause problems down the road.
https://www.orandsystems.com/Bm2DShow/show0.html This program lets you model stepped shafts. You get 30 unrestricted uses with the demo. I found that increasing the roller from .1875" long to .243" long so that it fits more snug inside of the .269" support span, causes a drastic reduction in the bending stress of the beam. I have pasted the program printout for both the .1875" long roller and a .243" long roller below. I will just use .010" thick delrin thrust washers on each side of the roller instead of .03 to .04" thick thrust washers. The highest bending stress seems to occur right where the 2mm shaft comes out of the 3/16" OD portion. With the .1875" long roller, the maximum bending stress is 84,130.45 PSI, but with the .243" long roller the maximum bending stress is reduced to 27,034.99 which surprised me. With the .1875" long roller, the center of the roller deflected by .0001" and the very ends of the 2mm OD end portions curled up by .0002". With the new .243" long roller, the center of the roller deflected by only 0.00005", and the very ends of the 2mm OD end portions deflected up by .0001". With the new longer roller, the first portion of the shaft is 2mm OD X .131" long, then the second portion is .1875" OD X .243" long, and the last portion is .2mm OD X .131" long. The question now becomes, will the pressed on 3/16" OD center portion act fairly close to a stepped shaft made from one solid piece as modeled by the program ? I do have one way to use a 1/8" OD shaft, but I must sacrifice the Igus plastic bushings. The roller and shaft is held in a yoke, I could make the yoke itself out of a bushing material, so the shaft turns right in the yoke instead of the plastic bushings. This gives me room for a 1/8" OD shaft. However, this is a high load oscillating application, and I can only lube the shaft once at assembly then never again. I am a Little concerned about wear. I hear 0-6 tool steel makes good bushings, and has a self lubricating graphitic property. The walls are so thin on the yoke I don't think I can harden it without cracking, so I would just have to lube the shaft at assembly, and hope for the best as far as wear is concerned. This thing is just intermittently oscillated by hand, so perhaps it would wear well. Here are the printouts from the beam design program. I would appreciate any other feedback anyone may have. If the new longer pressed on roller acts close to a one piece stepped shaft, I think I should be OK. NEW DESIGN WITH .243" LONG ROLLER BEAM LENGTH = 0.5047204 in MATERIAL PROPERTIES
CROSS-SECTION PROPERTIES
#2: from 0.1308602 in to 0.3738602 in
#3: from 0.3738602 in to 0.5047204 in
EXTERNAL CONCENTRATED FORCES
SUPPORT REACTIONS ***
Simple at 0.387 in
MAXIMUM DEFLECTION ***
MAXIMUM BENDING MOMENT ***
MAXIMUM SHEAR FORCE ***
MAXIMUM STRESS ***
ANALYSIS AT SPECIFIED LOCATIONS ***
Location = 0.07930511 in
Location = 0.1586102 in
Location = 0.2523602 in
Location = 0.3461102 in
Location = 0.4254153 in
Location = 0.5047204 in
OLD DESIGN WITH .1875" LONG ROLLER BEAM LENGTH = 0.5047204 in MATERIAL PROPERTIES
CROSS-SECTION PROPERTIES
#2: from 0.1586102 in to 0.3461102 in
#3: from 0.3461102 in to 0.5047204 in
EXTERNAL CONCENTRATED FORCES
SUPPORT REACTIONS ***
Simple at 0.387 in
MAXIMUM DEFLECTION ***
MAXIMUM BENDING MOMENT ***
MAXIMUM SHEAR FORCE ***
MAXIMUM STRESS ***
ANALYSIS AT SPECIFIED LOCATIONS ***
Location = 0.07930511 in
Location = 0.1586102 in
Location = 0.2523602 in
Location = 0.3461102 in
Location = 0.4254153 in
Location = 0.5047204 in
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