Making my own Light Weight Crank Pulley
Glad too see there's other people floating around here with machining experience, as a cnc machinist/programmer myself its always nice too fine people on car forums that understand what goes into actually making the parts for cars.
hey guys so i did a stress test on CAd of my pulley design. The test was applying a moment on the inside while holding the outside in place to simulate a sudden acceleration. I used a force of 250 lb/ft and the part results cam back with minimal deflection. Im thinking i can reduce weigh more on the part but i need an estimate max torque that i should gauge the design towards. i know my car is in the 185 range but i know you boosted folk are higher then that so what would be a sufficient number that our car could reach safely??
im going to do a 600 lb/ft test and see how it reacts and maybe use this value as a bass line with more then enough safety margin breather room for the higher output people
im going to do a 600 lb/ft test and see how it reacts and maybe use this value as a bass line with more then enough safety margin breather room for the higher output people
im not 100%...but i think cranswick has said with his custom turbo kit he's around 230hp if IIRC....i'm guessing tq is around the same range...
i couldnt think of an RA going more than 250hp or so unless it's hery heavily modified...laybe even different engine....so then the pulley probably wouldn't matter
i couldnt think of an RA going more than 250hp or so unless it's hery heavily modified...laybe even different engine....so then the pulley probably wouldn't matter
i wouls double it, i'm making 213 on paper, maybe 220 on a cool day at sea level...torque is almost the same, and hits sooner, i make 200 wheel ft-lbs @ like 2500rpm or less.
True, feels like his full torque hits hard at very low rpm, unlike OEM turbo cars.
btw, now when I think about Cranswick or Brent dumping the clutch while drag racing, here is an important design note: consider impact loads. Impact force magnifies the static load. Apply a load suddenly and it acts like about twice the normal amount gradually applied. Rule of thumb: to design for impact, use double the maximum static load.
Example, a locomotive rolling along the tracks applies a load, it's own weight, of 200 tons to the track. Now it approaches a bridge that spans a deep canyon. As soon as the fast-moving locomotive hits the bridge it impacts the trestle beneath it, applying a force to the track that is about twice it's static weight, about 400 tons.
Example: you grab the lid of a new jam jar and twist slowly. It won't budge. However, you hand it off to Grandma and she applies a quick twist. Off it comes, because she knew an impact load would double your slowly applied twisting force. (yes this actually works, try it).
btw, now when I think about Cranswick or Brent dumping the clutch while drag racing, here is an important design note: consider impact loads. Impact force magnifies the static load. Apply a load suddenly and it acts like about twice the normal amount gradually applied. Rule of thumb: to design for impact, use double the maximum static load.
Example, a locomotive rolling along the tracks applies a load, it's own weight, of 200 tons to the track. Now it approaches a bridge that spans a deep canyon. As soon as the fast-moving locomotive hits the bridge it impacts the trestle beneath it, applying a force to the track that is about twice it's static weight, about 400 tons.
Example: you grab the lid of a new jam jar and twist slowly. It won't budge. However, you hand it off to Grandma and she applies a quick twist. Off it comes, because she knew an impact load would double your slowly applied twisting force. (yes this actually works, try it).
Last edited by RalliartN; Mar 23, 2013 at 10:42 AM.
True, feels like his full torque hits hard at very low rpm, unlike OEM turbo cars.
btw, now when I think about Cranswick or Brent dumping the clutch while drag racing, here is an important design note: consider impact loads. Impact force magnifies the static load. Apply a load suddenly and it acts like about twice the normal amount gradually applied. Rule of thumb: to design for impact, use double the maximum static load.
Example, a locomotive rolling along the tracks applies a load, it's own weight, of 200 tons to the track. Now it approaches a bridge that spans a deep canyon. As soon as the fast-moving locomotive hits the bridge it impacts the trestle beneath it, applying a force to the track that is about twice it's static weight, about 400 tons.
Example: you grab the lid of a new jam jar and twist slowly. It won't budge. However, you hand it off to Grandma and she applies a quick twist. Off it comes, because she knew an impact load would double your slowly applied twisting force. (yes this actually works, try it).
btw, now when I think about Cranswick or Brent dumping the clutch while drag racing, here is an important design note: consider impact loads. Impact force magnifies the static load. Apply a load suddenly and it acts like about twice the normal amount gradually applied. Rule of thumb: to design for impact, use double the maximum static load.
Example, a locomotive rolling along the tracks applies a load, it's own weight, of 200 tons to the track. Now it approaches a bridge that spans a deep canyon. As soon as the fast-moving locomotive hits the bridge it impacts the trestle beneath it, applying a force to the track that is about twice it's static weight, about 400 tons.
Example: you grab the lid of a new jam jar and twist slowly. It won't budge. However, you hand it off to Grandma and she applies a quick twist. Off it comes, because she knew an impact load would double your slowly applied twisting force. (yes this actually works, try it).
I understand where your coming from and its something i have thought about before aswell. The way i see it tho is since its a pulley driving a belt there isnt nearly as much "sudden" load as your train example or say the force that the gears or tires would see suddenly. I would think the belt and then the tensioner would help to diminish alot of this sudden impact. Also since the car is presumably already running at idle or during these harder drag launches at say 3500 rpm then the pulley and all the aux belt components are already turning and have inertia. yes there would be maybe a bit of a kick but again not nearly as sever as what your thinking. What are your opinions to that?
edit1: from what i remember at work the hardest conditions for the crank pulley were during a stall and or an engine start, but i dont think they did tests for drag launches :P
And i really do appreciate all the criticism, comments, ideas that you guys make as this really helps to add to my project and give me a more thorough understanding. Also since im on my own it helps to bring up new ideas that i may have missed or overlooked.
Last edited by RallyartRob; Mar 24, 2013 at 06:15 AM.
Yes, your thread does attract the attention of "Mr. Helper" (
remember the Rodney Dangerfield movie Back to School when Rodney was in Sam Kinison's class?)
My opinion is, when in doubt or guessing, and when weight is not a critical consideration like it is in aerospace design, and if its within cost and size constraints, then design the part to withstand the absolute worst case. We could design using statistical probability, which basically means if we figure out that the probability of a severe loading event occuring during use is outside the plus 3 sigma range, we don't include this extreme loading scenario. That is acceptable design practice. Again, though, for typical commercial use, I'd say as a unique small supplier you use the very worst case no matter how unlikely it might be. Showing this consideration would help to defeat a lawsuit when you have to prove your design in court against an irate customer who broke your part.
What I'm saying then is I'd test using the impact load of 2 times the static load, just to be sure.
The other consideration is fatigue and also the effect of material degradation due to oils fuel etc. That's a lot of testing though. I think just mimicking the OEM materials or bettering them would be fine here.
Ah, engineering for the public is more than just "making a part", as you are aware.
p.s. Rob, keep up the good effort. I'm not here for attention or to criticize you, I'm an older guy who has made a living doing this stuff and I'm simply chiming in with some ideas.
remember the Rodney Dangerfield movie Back to School when Rodney was in Sam Kinison's class?)My opinion is, when in doubt or guessing, and when weight is not a critical consideration like it is in aerospace design, and if its within cost and size constraints, then design the part to withstand the absolute worst case. We could design using statistical probability, which basically means if we figure out that the probability of a severe loading event occuring during use is outside the plus 3 sigma range, we don't include this extreme loading scenario. That is acceptable design practice. Again, though, for typical commercial use, I'd say as a unique small supplier you use the very worst case no matter how unlikely it might be. Showing this consideration would help to defeat a lawsuit when you have to prove your design in court against an irate customer who broke your part.
What I'm saying then is I'd test using the impact load of 2 times the static load, just to be sure.
The other consideration is fatigue and also the effect of material degradation due to oils fuel etc. That's a lot of testing though. I think just mimicking the OEM materials or bettering them would be fine here.
Ah, engineering for the public is more than just "making a part", as you are aware.
p.s. Rob, keep up the good effort. I'm not here for attention or to criticize you, I'm an older guy who has made a living doing this stuff and I'm simply chiming in with some ideas.
Last edited by RalliartN; Mar 24, 2013 at 08:03 AM.
Yes, your thread does attract the attention of "Mr. Helper" ( remember the Rodney Dangerfield movie Back to School when Rodney was in Sam Kinison's class?)
My opinion is, when in doubt or guessing, and when weight is not a critical consideration like it is in aerospace design, and if its within cost and size constraints, then design the part to withstand the absolute worst case. We could design using statistical probability, which basically means if we figure out that the probability of a severe loading event occuring during use is outside the plus 3 sigma range, we don't include this extreme loading scenario. That is acceptable design practice. Again, though, for typical commercial use, I'd say as a unique small supplier you use the very worst case no matter how unlikely it might be. Showing this consideration would help to defeat a lawsuit when you have to prove your design in court against an irate customer who broke your part.
What I'm saying then is I'd test using the impact load of 2 times the static load, just to be sure.
The other consideration is fatigue and also the effect of material degradation due to oils fuel etc. That's a lot of testing though. I think just mimicking the OEM materials or bettering them would be fine here.
Ah, engineering for the public is more than just "making a part", as you are aware.
p.s. Rob, keep up the good effort. I'm not here for attention or to criticize you, I'm an older guy who has made a living doing this stuff and I'm simply chiming in with some ideas.
remember the Rodney Dangerfield movie Back to School when Rodney was in Sam Kinison's class?)My opinion is, when in doubt or guessing, and when weight is not a critical consideration like it is in aerospace design, and if its within cost and size constraints, then design the part to withstand the absolute worst case. We could design using statistical probability, which basically means if we figure out that the probability of a severe loading event occuring during use is outside the plus 3 sigma range, we don't include this extreme loading scenario. That is acceptable design practice. Again, though, for typical commercial use, I'd say as a unique small supplier you use the very worst case no matter how unlikely it might be. Showing this consideration would help to defeat a lawsuit when you have to prove your design in court against an irate customer who broke your part.
What I'm saying then is I'd test using the impact load of 2 times the static load, just to be sure.
The other consideration is fatigue and also the effect of material degradation due to oils fuel etc. That's a lot of testing though. I think just mimicking the OEM materials or bettering them would be fine here.
Ah, engineering for the public is more than just "making a part", as you are aware.
p.s. Rob, keep up the good effort. I'm not here for attention or to criticize you, I'm an older guy who has made a living doing this stuff and I'm simply chiming in with some ideas.
Im not going to actually put the part into production i dont think because of the overhead cost and what not. Ill probably just do a run of 2-3 and keep one for my "wall".
As for teh design from what i know the oem part is aluminum inner and outer with the rubber piece. my parts is pretty much an identical copy minus the rubber part, all other thickness are the same size, and ive removed excess material form the inner "spoke" area to reduce weight like all the other companies do. My concern was yeah it will do fine on my car which is in the 180 range hp/tq but say cranswick does it on his turbo build will it still be ok? hes probably in the 230hp 200 torque range just guessing maybe less. Now ive tested at 250 ft/lbs and the CAD test came back with no problems to my knowledge but that CAD its not always real world.
As for gas and chemicals im not overly concerned because ive seen alot of untreated aluminum pulleys on the market.
Just reviving this to clarify (in my mind anyway): can the stock crank pulley with the rubber insert be replaced by an all-metal underdrive pulley and not hurt the engine?
Yes.
Reference: http://www.grimmspeed.com/catalog/pr...roducts_id=230
Summary: The Subaru factory says the pulley with the rubber insert does not act like a harmonic damper. It is simply a pulley. Period.
The rubber is put there to minimize the bit of vibration and harshness felt by the driver due to the accessories driven by the belt.
Real harmonic dampers are the fluid filled canisters that a crank pulley bolts to, used mostly in high power 6 and 8 cylinder cars. The RA doesn't have that.
So, I will continue to run the underdrive pulley with no fear. I had re-installed the OEM pulley last weekend, and in back-to-back comparisons the underdrive pulley'd engine does rev quicker, it is obvious.
Yes.
Reference: http://www.grimmspeed.com/catalog/pr...roducts_id=230
Summary: The Subaru factory says the pulley with the rubber insert does not act like a harmonic damper. It is simply a pulley. Period.
The rubber is put there to minimize the bit of vibration and harshness felt by the driver due to the accessories driven by the belt.
Real harmonic dampers are the fluid filled canisters that a crank pulley bolts to, used mostly in high power 6 and 8 cylinder cars. The RA doesn't have that.
So, I will continue to run the underdrive pulley with no fear. I had re-installed the OEM pulley last weekend, and in back-to-back comparisons the underdrive pulley'd engine does rev quicker, it is obvious.
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