Balance shafts
01-21-2003, 02:33 AM
#16
Burning Brakes
Join Date: Nov 2001
Location: San Diego
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Matt,
I don't have the info for the source here at the house, but I'll get it and post it to you tomorow. In short, its hard , very hard when it cures. Infact, Total Performance in Santee uses the stuff to fill old o ring grooves in heads when they have to move the wire, and it holds with no failure even under those extreme conditions. I'm sure they use it for other fixes as well, but its good stuff. Its not cheap, around 70.00 a small bottle (the amount of about 4.00 worth of JB weld). Total performance build top fuel drag engines, so I kinda trust them, and to date, no leaks in the areas of the oil drains in the balance shafts :-)
Take Care!
I don't have the info for the source here at the house, but I'll get it and post it to you tomorow. In short, its hard , very hard when it cures. Infact, Total Performance in Santee uses the stuff to fill old o ring grooves in heads when they have to move the wire, and it holds with no failure even under those extreme conditions. I'm sure they use it for other fixes as well, but its good stuff. Its not cheap, around 70.00 a small bottle (the amount of about 4.00 worth of JB weld). Total performance build top fuel drag engines, so I kinda trust them, and to date, no leaks in the areas of the oil drains in the balance shafts :-)
Take Care!
01-21-2003, 09:01 AM
#18
Race Director
I think the discussion on the balance-shafts always seems a little incomplete because it is has people presenting their personal experiences (empirical data) without ever delving into the nitty-gritty details of the physics behind it all. As such, there are underlying principles that are ignored. One thing to keep in mind is that we have by far the largest displacement 4-cylinder powerplants to ever be used in a production sports car. With the heaviest pistons and rods, not to mention flywheel and crank. While you can try as much as possible to get overall primary balance with lightened and matched parts, you can't achieve static primary balance on an individual per-journal basis with each single rod & piston combo if you lighten the crank.
Primary forces are inertia forces created by the acceleration (+ and -) of the piston assembly mass caused by the rotating crankpin’s projected motion along the line of stroke due to the reciprocating motion of the piston assembly.
Secondary forces are those inertia forces caused by the projected motion perpendicular to the line of stroke caused by the rotating motion of the connecting rod._ That is, the secondary force is due to the additional piston acceleration (both + and -) produced by the rotating crankpin increasing or decreasing the inclination of the connecting rod to the line of stroke._ During the first 90 deg of crank rotation, this secondary movement of the connecting rod is away from the line of stroke, thus adding to the piston movement while during the second 90 deg of crank rotation, this secondary movement of the connecting rod is toward the line of stroke, thus subtracting from the distance the piston moves._ Furthermore, secondary forces increase and decrease magnitude at twice the frequency of the primary force, but their maximum values are only about 1/4 of the dominating primary force.
_
Inertia Force F = Q1 Cos q + Q2 Cos 2q
where _ _ _ _ q = angle between crankpin and cylinder axis
Q1 Cos q represents the Primary inertia force (1st Harmonic), occurring at engine speed due to Cos q term
Q2 Cos 2q represents the Secondary inertia force (2nd Harmonic), at twice engine speed due to Cos 2q_
Given a 4 cylinder with the following crank configuration, the following moments can be defined:
X = distance between crank throws,
+3X/2 = lever arm to cylinder 1
+X/2 = lever arm to cylinder 2
–X/2 = lever arm to cylinder 3
–3X/2 = lever arm to cylinder 4
Sketching the crank in some general position and sum the forces as follows:
As shown above, the sum of the primary forces is zero._ Completing the analysis, a total primary analysys would appear below:
Which agrees with the graphical inspection and is valid for all crank positions. We can see that all primary forces and moments for this 4 cylinder engine sum to zero, and secondary moments sum to zero as well._
However, a second order force unbalance remains at twice engine speed.
This diagram illustrates the primary and secondary forces again._ The full wide arrows represent the primary forces and the narrow half arrows represent the secondary forces._ Note that the primary forces are actually not exactly balanced but are very close, so in reality there still exists some vertical shake._ The primary couples are absorbed by the rigidity of the crankshaft material, while the four secondary forces definitely cause vehicle shake.
This diagram demonstrates how the primary forces are canceled out, while the 4 secondary forces are additive at 90 deg intervals, contributing to horizontal shake._ On engines of less than 2 liters, this shake can be tolerated; however, on larger displacement engines this secondary imbalance must be corrected by using twin countershafts.
The weight and rotation direction of the countershaft is critical for proper operation._ These two countershafts must have weights Bl and Br equivalent in magnitude to the secondary inertia forces of all 4 cylinders._ Bl+Br = 4Fs, Fs = secondary force._ The countershafts revolve at twice crankshaft speed using a 2:1 belt & sprocket configuration and rotate counter to each other (Bl clockwise, Br counterclockwise) and are timed so that they counteract +Fs at 0 deg (TDC) and 180 deg ATDC, and –FS at mid-crank position (90 deg ATDC)._
Secondary balance is accomplished by the weights facing in the opposite direction to these secondary forces when in the vertical plane. In the horizontal plane the weights will oppose each other, with one facing inward (135 deg ATDC) while the other faces outward (45 deg ATDC).
So as you can see, while primary balance can be ahieved statically and dynamically, the secondary dynamic balance is not possible without balance shafts. Maybe if we had a four-cylinder with twin counter-rotating cranks perhaps...
While you may diminish the secondary shake through lightening parts and good engine-mounts, the vibration is still there, just not noticeable.
------------------------------------------------
reference: Advanced Engine Technology, 2nd-Edition Heisler 1995
Primary forces are inertia forces created by the acceleration (+ and -) of the piston assembly mass caused by the rotating crankpin’s projected motion along the line of stroke due to the reciprocating motion of the piston assembly.
Secondary forces are those inertia forces caused by the projected motion perpendicular to the line of stroke caused by the rotating motion of the connecting rod._ That is, the secondary force is due to the additional piston acceleration (both + and -) produced by the rotating crankpin increasing or decreasing the inclination of the connecting rod to the line of stroke._ During the first 90 deg of crank rotation, this secondary movement of the connecting rod is away from the line of stroke, thus adding to the piston movement while during the second 90 deg of crank rotation, this secondary movement of the connecting rod is toward the line of stroke, thus subtracting from the distance the piston moves._ Furthermore, secondary forces increase and decrease magnitude at twice the frequency of the primary force, but their maximum values are only about 1/4 of the dominating primary force.
_
Inertia Force F = Q1 Cos q + Q2 Cos 2q
where _ _ _ _ q = angle between crankpin and cylinder axis
Q1 Cos q represents the Primary inertia force (1st Harmonic), occurring at engine speed due to Cos q term
Q2 Cos 2q represents the Secondary inertia force (2nd Harmonic), at twice engine speed due to Cos 2q_
Given a 4 cylinder with the following crank configuration, the following moments can be defined:
X = distance between crank throws,
+3X/2 = lever arm to cylinder 1
+X/2 = lever arm to cylinder 2
–X/2 = lever arm to cylinder 3
–3X/2 = lever arm to cylinder 4
Sketching the crank in some general position and sum the forces as follows:
As shown above, the sum of the primary forces is zero._ Completing the analysis, a total primary analysys would appear below:
Which agrees with the graphical inspection and is valid for all crank positions. We can see that all primary forces and moments for this 4 cylinder engine sum to zero, and secondary moments sum to zero as well._
However, a second order force unbalance remains at twice engine speed.
This diagram illustrates the primary and secondary forces again._ The full wide arrows represent the primary forces and the narrow half arrows represent the secondary forces._ Note that the primary forces are actually not exactly balanced but are very close, so in reality there still exists some vertical shake._ The primary couples are absorbed by the rigidity of the crankshaft material, while the four secondary forces definitely cause vehicle shake.
This diagram demonstrates how the primary forces are canceled out, while the 4 secondary forces are additive at 90 deg intervals, contributing to horizontal shake._ On engines of less than 2 liters, this shake can be tolerated; however, on larger displacement engines this secondary imbalance must be corrected by using twin countershafts.
The weight and rotation direction of the countershaft is critical for proper operation._ These two countershafts must have weights Bl and Br equivalent in magnitude to the secondary inertia forces of all 4 cylinders._ Bl+Br = 4Fs, Fs = secondary force._ The countershafts revolve at twice crankshaft speed using a 2:1 belt & sprocket configuration and rotate counter to each other (Bl clockwise, Br counterclockwise) and are timed so that they counteract +Fs at 0 deg (TDC) and 180 deg ATDC, and –FS at mid-crank position (90 deg ATDC)._
Secondary balance is accomplished by the weights facing in the opposite direction to these secondary forces when in the vertical plane. In the horizontal plane the weights will oppose each other, with one facing inward (135 deg ATDC) while the other faces outward (45 deg ATDC).
So as you can see, while primary balance can be ahieved statically and dynamically, the secondary dynamic balance is not possible without balance shafts. Maybe if we had a four-cylinder with twin counter-rotating cranks perhaps...
While you may diminish the secondary shake through lightening parts and good engine-mounts, the vibration is still there, just not noticeable.
------------------------------------------------
reference: Advanced Engine Technology, 2nd-Edition Heisler 1995
Last edited by Danno; 09-15-2003 at 02:46 PM.
01-21-2003, 11:12 AM
#19
Addict
Rennlist Member
Rennlist Member
</font><blockquote><font size="1" face="Verdana,Tahoma,Arial,Helvetica,Geneva">quote:</font><hr /><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva"> While you may diminish the secondary shake through lightening parts and good engine-mounts, the vibration is still there, just not noticeable.
</font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">So, what problems do I need to on the look out for?
</font><hr /></blockquote><font size="2" face="Verdana,Tahoma,Arial,Helvetica,Geneva">So, what problems do I need to on the look out for?
