ARP Reciprocating Weight Formula

[soup]

I've been wracking my brain for the past two days trying to figure out this damn equation for the reciprocating weight, or oscillating force on the rod bolts:

So let's say I use these rough numbers:
Piston MASS: 310grams = 0.7lbs
Rod MASS: 540grams = 1.2lbs
Stroke: 3.852"
RPM: 6000RPM*2*pi/60

That gets me: 1,555,528 lb-in/second squared
then 129627 lb-ft/s^2
then divide by 32.2 ft/s^2 and I get an oscillating force of 4000 lbs!?! Seems a little light. ARP stated a rough value of 20,000 lbs at the same rpm on a different engine.

ARP states that the oscillating force is PROPORTIONAL to the above calc. So pretty much it's useless and I need to hit up some textbooks and figure it out on my own. Or just multiply it by 4 and hope the prof doesn't notice

Have any thoughts?

 

[92nsx]

Have any thoughts?

You have to much time on your hands. That is my thoughts

 

[grimreaper]

^ lol,
i compliment you on learning more then the "basics" though! I bet figgie could do this in his sleep... or a few others

 

[figgie]

that because you are changing the mass.

WEIGHT (ie lbs) is NOT, nor will it ever be MASS

you do the math with MASS, then the answer will need to be multiplied/divided etc to get the correct force measurement.

 

[adampecush]

why didn't you square the RPM value in your calculations like the equation states?

 

[sneakypete]

as figgie stated... you need to use pounds mass (lbm), not pounds force (lbf)

 

[soup]

RPM was squared and i divided it later by 32.2 for gravity . My imperial units are a little rusty, pound-force. slug foot blob whatever. Why can't you guys just go metric, geez.

who wants the biggest geek prize ?

 

[figgie]

RPM was squared and i divided it later by 32.2 for gravity . My imperial units are a little rusty, pound-force. slug foot blob whatever. Why can't you guys just go metric, geez.

who wants the biggest geek prize ?

32.2 what? Last I checked Earth's Gravity is 9.8 m/s^2 which equates to 32.15 ft/s^2 in the imperial units. Event hough your whole number is correct. Always go down to the hundreths.

No need for imperial in physics! Physics is ALL metric.

ie Gravity on earth = 9.8 m/s^2 = 32.15 ft/s^2
Mass always measured in grams
distance = meters and its derivatives (mm, cm, km, etc)
Speed = meters per second or it deravitives (mm/s, cm/s, km/s)
Energy = Netwons or Joules depending on what is being solved.

 

[sneakypete]

lol... us americans like to be different than the rest of the world. it doesnt really matter if its in metric or standard, as long as if your units are consistent. and rounding the gravity to 32.2 ft/s^2 is accurate enough for this calculation
-pete