Rod bolt load calculation
Discussion
Hi,
A basic calculation will require the determination of the reciprocating load, approx half the rotating load and the load from the interferance fit of the big end bearing. A safety factor of two is normally used to account for initial bending of the bolt on installation and in use.
The math in the ARP book is not entirely complete. Your best bet if you cannot work out the above loads is to contact ARP and they can run the calculations for you.
A more detailed approach can be done using the German VDI2230 standard.
A basic calculation will require the determination of the reciprocating load, approx half the rotating load and the load from the interferance fit of the big end bearing. A safety factor of two is normally used to account for initial bending of the bolt on installation and in use.
The math in the ARP book is not entirely complete. Your best bet if you cannot work out the above loads is to contact ARP and they can run the calculations for you.
A more detailed approach can be done using the German VDI2230 standard.
the problem with the various formulas dotted around the net, on the arp site and forums etc. is that it's never made clear what units everything is in and different sides of the pond like to use different units and mix things up.
at the end of the day, i just want to know if ARP2000's are enough to let a 408g piston and ~600g rod rev to 9000RPM with a 75.65mm stroke. i'd also be interested in what a standard rod bolt (assuming 150,000psi) would rev to with a 717g rod before it let go.
at the end of the day, i just want to know if ARP2000's are enough to let a 408g piston and ~600g rod rev to 9000RPM with a 75.65mm stroke. i'd also be interested in what a standard rod bolt (assuming 150,000psi) would rev to with a 717g rod before it let go.
Edited by nedge2k on Thursday 9th February 00:36
Edited by Tuscan Rat on Thursday 9th February 09:11
nedge2k said:
the problem with the various formulas dotted around the net, on the arp site and forums etc. is that it's never made clear what units everything is in and different sides of the pond like to use different units and mix things up.
at the end of the day, i just want to know if ARP2000's are enough to let a 408g piston and ~600g rod rev to 9000RPM with a 75.65mm stroke. i'd also be interested in what a standard rod bolt (assuming 150,000psi) would rev to with a 717g rod before it let go.
and it doesn't even occur to you that the actual diameter and thread profile of the rod bolt might be an important element in the calculation? Hmmmm.at the end of the day, i just want to know if ARP2000's are enough to let a 408g piston and ~600g rod rev to 9000RPM with a 75.65mm stroke. i'd also be interested in what a standard rod bolt (assuming 150,000psi) would rev to with a 717g rod before it let go.
Pumaracing said:
and it doesn't even occur to you that the actual diameter and thread profile of the rod bolt might be an important element in the calculation? Hmmmm.
it does actually but as i'm trying to work this out for myself, i saw no point in mentioning it. besides, I had assumed anyone with knowledge of rod bolt load formulas would know that most forged rods come with an off the shelf 3/8 UNF ARP2000 bolt with an 8mm shank. generally speaking anyway.nedge2k said:
it does actually but as i'm trying to work this out for myself, i saw no point in mentioning it. besides, I had assumed anyone with knowledge of rod bolt load formulas would know that most forged rods come with an off the shelf 3/8 UNF ARP2000 bolt with an 8mm shank. generally speaking anyway.
Ive seen many engines with forged rods that didnt use 3/8" UNF, so really dont think that applies.Ive seen some with 7/16, 3/8, M9, M8
nedge2k said:
I had assumed anyone with knowledge of rod bolt load formulas would know that most forged rods come with an off the shelf 3/8 UNF ARP2000 bolt with an 8mm shank. generally speaking anyway.
Looking at your weights I would say a 7/16 bolt is more appropriate. If you want to go with 3/8 I'd use the Custom Age 625 material.I didn't mean my previous post to be offensive in any way.
Ok, think I've figured this out:
SO
Piston acceleration (m/s^2) = (((N^2 x L)/54725) x (1 + (1/(2A))) / 10.7639) = ((((9000^2) * 75.65)/54725) * (1 + (1/(2*(145/75.65)))) / 10.7639) = 13116.1419 m/s^2
Where:
N = RPM
L = stroke (mm)
A = rod length / stroke (mm)
Reciprocating & Rotational Mass (kg) = ((P + (R/3) + (R/3))/1000) = ((408 + (717/3) + (717/3))/1000) = 0.886 kg
Where:
P = piston weight (g)
R = rod weight (g)
Force (newtons) = Reciprocating & rotational mass (kg) x Piston Acceleration (m/s^2) = (13116.1419*0.886) = 11620.90172N
Pressure (psi) = (Force (newtons) / Bolt diameter (mm)) x 145.037 (to convert n/mm^2 to psi) = ((11620.90172/8)*145.037)/2 = 105341.2952 psi
Then I guess you'd divide by two to give your per-bolt rating.
...which all means that an 8mm shank rod bolt would need to take ~77,000PSI @ 9000RPM for a stroke of 75.65mm with a 145mm rod, 408g piston and 717g rod or ~72,000PSI for a 600g rod.
Right?
Which now makes ~105,000PSI per bolt with the 717g rod and ~96,000PSI per bolt for the 600g rod. Double that up for your safety factor of 2 (as ARP suggest) and it turns out ARP2000's 200-220,000PSI rating should be fine
SO
Piston acceleration (m/s^2) = (((N^2 x L)/54725) x (1 + (1/(2A))) / 10.7639) = ((((9000^2) * 75.65)/54725) * (1 + (1/(2*(145/75.65)))) / 10.7639) = 13116.1419 m/s^2
Where:
N = RPM
L = stroke (mm)
A = rod length / stroke (mm)
Reciprocating & Rotational Mass (kg) = ((P + (R/3) + (R/3))/1000) = ((408 + (717/3) + (717/3))/1000) = 0.886 kg
Where:
P = piston weight (g)
R = rod weight (g)
Force (newtons) = Reciprocating & rotational mass (kg) x Piston Acceleration (m/s^2) = (13116.1419*0.886) = 11620.90172N
Pressure (psi) = (Force (newtons) / Bolt diameter (mm)) x 145.037 (to convert n/mm^2 to psi) = ((11620.90172/8)*145.037)/2 = 105341.2952 psi
Then I guess you'd divide by two to give your per-bolt rating.
Right?
Added rotational mass & calcs
Which now makes ~105,000PSI per bolt with the 717g rod and ~96,000PSI per bolt for the 600g rod. Double that up for your safety factor of 2 (as ARP suggest) and it turns out ARP2000's 200-220,000PSI rating should be fine
Edited by nedge2k on Friday 10th February 09:23
nedge2k said:
Ok, think I've figured this out:
SO
Piston acceleration (m/s^2) = (((N^2 x L)/54725) x (1 + (1/(2A))) / 10.7639)
Where:
N = RPM
L = stroke (mm)
A = rod length / stroke (mm)
Reciprocating mass (kg) = ((P + (R/3))/100)
Where:
P = piston weight (g)
R = rod weight (g)
Force (newtons) = Reciprocating mass (kg) x Piston Acceleration (m/s^2)
Pressure (psi) = (Force (newtons) / Bolt diameter (mm)) x 145.037 (to convert n/mm^2 to psi)
Then I guess you'd divide by two to give your per-bolt rating.
...which all means that an 8mm shank rod bolt would need to take ~77,000PSI @ 9000RPM for a stroke of 75.65mm with a 145mm rod, 408g piston and 717g rod or ~72,000PSI for a 600g rod.
Right?
I am having a little trouble getting my calculations to agree. Can you show me your math?SO
Piston acceleration (m/s^2) = (((N^2 x L)/54725) x (1 + (1/(2A))) / 10.7639)
Where:
N = RPM
L = stroke (mm)
A = rod length / stroke (mm)
Reciprocating mass (kg) = ((P + (R/3))/100)
Where:
P = piston weight (g)
R = rod weight (g)
Force (newtons) = Reciprocating mass (kg) x Piston Acceleration (m/s^2)
Pressure (psi) = (Force (newtons) / Bolt diameter (mm)) x 145.037 (to convert n/mm^2 to psi)
Then I guess you'd divide by two to give your per-bolt rating.
...which all means that an 8mm shank rod bolt would need to take ~77,000PSI @ 9000RPM for a stroke of 75.65mm with a 145mm rod, 408g piston and 717g rod or ~72,000PSI for a 600g rod.
Right?
On the engines that I deal with a 408g piston seem light. Does this include the pin any locks and the rings?
Stan
PS - Reciprocating mass (kg) = ((P + (R/3))/100) <--- Should not that be 1000
Edited by Stan Weiss on Friday 10th February 06:41
Stan Weiss said:
I am having a little trouble getting my calculations to agree. Can you show me your math?
On the engines that I deal with a 408g piston seem light. Does this include the pin any locks and the rings?
Stan
PS - Reciprocating mass (kg) = ((P + (R/3))/100) <--- Should not that be 1000
You're right, it was late and I was half asleep and forgot the last 0. Also forgot to weigh the rings with the piston - although it was in it's bubble wrap bag with pin and clips so not far off :POn the engines that I deal with a 408g piston seem light. Does this include the pin any locks and the rings?
Stan
PS - Reciprocating mass (kg) = ((P + (R/3))/100) <--- Should not that be 1000
Edited by Stan Weiss on Friday 10th February 06:41
I also wonder whether rotational mass should be included - which would be roughly half of the lower 2 thirds of the rod?
Amended the calc anyway, can't hurt.
Edited by nedge2k on Friday 10th February 08:57
Stan Weiss said:
I am having a little trouble getting my calculations to agree.
Probably because every part of the calculation is wrong. The peak piston acceleration is out by a factor of nearly 4, the crank pin acceleration is not the same as the piston acceleration at TDC anyway so the rotating load calculations are wrong and need to be done separately from the reciprocating ones and the bolt psi part has a factor of Pi missing and uses the wrong bolt shank diameter.Other than that.....
Pumaracing said:
Probably because every part of the calculation is wrong. The peak piston acceleration is out by a factor of nearly 4, the crank pin acceleration is not the same as the piston acceleration at TDC anyway so the rotating load calculations are wrong and need to be done separately from the reciprocating ones and the bolt psi part has a factor of Pi missing and uses the wrong bolt shank diameter.
Other than that.....
So you have plenty of time to criticise but none to actually help Other than that.....
nedge2k said:
So you have plenty of time to criticise but none to actually help
nedge2k said:
.. but as i'm trying to work this out for myself.
So you need to make your mind up as to whether you actually really want to work this out for yourself in which case I've told you what you've got wrong so far which is all of it or whether you want an older brother or sister to help before you put someone's eye out.Strangely your errors to date albeit huge are somewhat self compensating but I suppose the law of averages would predict that. An infinite number of monkeys tapping away at typewriters etc.
Pumaracing said:
So you need to make your mind up as to whether you actually really want to work this out for yourself
I wanted to work it out for myself, with a formula - not necessarily have to take a course in physics or have someone run the numbers for me.Pumaracing said:
or whether you want an older brother or sister to help before you put someone's eye out.
..and which are you, Arthur or Marthur?Pumaracing said:
Strangely your errors to date albeit huge are somewhat self compensating but I suppose the law of averages would predict that.
So, basically, I'm right.Pumaracing said:
An infinite number of monkeys tapping away at typewriters etc.
Would that be your normal approach to engine building?Edited by nedge2k on Friday 10th February 13:54
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