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Thread: Clutch Slip?

  1. #1
    Tube Master's Avatar
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    Clutch Slip?

    Do you realy need the clutch to slip at all when running a BOV?


  2. #2
    CRAZEDRXP's Avatar
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    having seen a gear driven supercharger in other applications in detail I can offer you an explanation on this one:

    When the throttle plate closes and the SC continues to spin it builds up pressure in the plenum. That pressure gets to a certain point and then has to go somewhere. With no BOV it will go back through the SC. Given the size of the cavity and the volume of air the sc moves (especially with the other impellor in place) this happens quick. This is bad simply for the reason that the gearing in that SC is designed to go one direction (to compress). Air coming back through it will try to create resistance against the drive gearing for the SC which is where the clutch comes into play. It will disengage under pressure allowing the air to escape, etc. However the problem is there is a certain amount of negative resistance required to engage the clutch which could also be classified as wear as that additional air tries to force the SC in theopposite direction prior to the engagement of the clutch. Not to mention no matter how precise of a device at the speeds SC's spin to compress even 1 additional atmosphere (8psi +/-) it's almost impossible to have a clean engagement.

    That negative resistance working against the gear system (in this case trying to make the SC slow down much more rapidly than it would traditionally) is putting strain on the system in the opposite direction it was designed to be strained. You should also realize that in forced induction supercharging traditionally puts considerably more strain on an engine than turboing...

    The BOV assembly would eliminate that by releasing that air that creates the strain befoe (or maybe even at) the same point the clutch is designed to engage. No matter what happens with that air supply, if the SC is gear driven it's going to spin very closely to the ratio it was designed to spin at based off the gearing from the crank. If there's air in there trying to force it's way through that's just going to create even more strain on the system. That's the reason for the clutch assembly in the first place but the first year design wasn't really up to the task it seems.

    Just my 2 cents, take it or leave it. I've yet to see a high hp application (which given it's displacement the RXP certainly is) of forced induction that doesn't use some type of method for bypassing boost. And most of those applications use belts which would seem to be much more capable of handling negative resistance as they'd just slip. No intermeshing teeth to worry about...

  3. #3
    Except, it's not just the boost that can cause the clutch to slip. I’m not saying CRAZEDRXP is wrong or right with his analysis of the situation. But consider this.

    “The motion of an aircraft through the air can be explained and described by physical principals discovered over 300 years ago by Sir Isaac Newton. Newton worked in many areas of mathematics and physics. He developed the theories of gravitation in 1666, when he was only 23 years old. Some twenty years later, in 1686, he presented his three laws of motion in the "Principia Mathematica Philosophiae Naturalis." The laws are shown above, and the application of these laws to aerodynamics are given on separate slides.

    Newton's first law states that every object will remain at rest or in uniform motion in a straight line unless compelled to change its state by the action of an external force. This is normally taken as the definition of inertia. The key point here is that if there is no net force acting on an object (if all the external forces cancel each other out) then the object will maintain a constant velocity. If that velocity is zero, then the object remains at rest. If an external force is applied, the velocity will change because of the force.

    The second law explains how the velocity of an object changes when it is subjected to an external force. The law defines a force to be equal to change in momentum (mass times velocity) per change in time. Newton also developed the calculus of mathematics, and the "changes" expressed in the second law are most accurately defined in differential forms. (Calculus can also be used to determine the velocity and location variations experienced by an object subjected to an external force.) For an object with a constant mass m, the second law states that the force F is the product of an object's mass and its acceleration a:

    F = m * a

    For an external applied force, the change in velocity depends on the mass of the object. A force will cause a change in velocity; and likewise, a change in velocity will generate a force. The equation works both ways. "


    If we eliminate the effect of boost upon the SC impeller, we still have to deal with the inertial forces involved. Since the SC is gear driven, there is an extraordinary amount of strain on the relatively small SC impeller shaft. When there are sudden and rapid changes in rpm (for example when the pump comes unhooked at WOT) the SC gear and shaft bear all the strain of the impeller’s inertia. The clutch is designed to “soften” the strain placed on the shaft.

    Basically, there are two trains of thought out there right now. Less slip with a BOV, and less slip without a BOV.

    The believers of the “less slip with a BOV” theory believe that the majority of the slip is caused by excessive boost. CRAZEDRXP has explained this theory well enough that I won’t go into it any further.

    The believers of the “less slip without the BOV” believe that the majority of the slip is caused by inertial forces during rapid/sudden rpm changes. This theory believes that the excessive boost that CRAZEDRXP described under deceleration actually helps slow the impeller reducing the strain on the sc impeller shaft, thus reducing strain on the clutch. ROTAX (the engine manufacturer) subscribes to this theory.

    So the real question is, was the SC clutch designed to handle excessive boost? Or was it designed to protect the SC shaft from the inertial forces of the SC impeller. Both theories have valid points IMO. And I don’t have the answer as to which one is “more correct”.

  4. #4
    oh yeah, I forgot to mention one thing.

    I haven't heard of a successful attempt at eliminating or permanantly locking up the sc clutch. Every one I've heard of so far has ended up in a broken sc impeller shaft or a shattered drive gear.

  5. #5
    Bullit's Avatar
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    Very good Thomas. Will a few pounds of air pressure contained with hose clamps break shafts and gears? I can't see that happening. Can the energy stored in an object (even as lite as the impeller) spinning at up to 45,000 rpm do that type of damage or cause the clutch to generate the heat needed to destroy the ceramic washers? I think so.
    Someone else can work out that math problem, I am going to Lake Sam Rayburn to ride.

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