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Thread: ULT Prototype History

  1. #1
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    ULT Prototype History

    Does anyone know the history on the ULT prototypes and testing? I searched through many of Tom's product updates prior to its release, but I couldn't find what I was looking for. Apparently, there were mechanical bounce issues that violated tournament rules.

    I am wondering why, with the ULT as released, the ULT on/off pin head diameter is the same as the RT on/off shaft diameter. Since one is only pulling against the pressure X surface area of the shaft diameter (exposed bottom surface of the head diameter cancels out most of the top surface), why wasn't the head diameter of the ULT on/off pin left the same as the RT on/off, and only the shaft diameter reduced? This would give a harder trigger reset but still provide the same trigger pull weight.

    Part of me thinks the shim adjustments would be able to cancel out mechanical bounce.

    The other part of me thinks the difference between trigger pull force and return force would have been too large to cancel out the Mechanical bounce. In other words, with so much force initially pushing the trigger forward just until air can squeeze by the top on/off oring to recharge the dump chamber, and then such a soft trigger pull required to get it to shoot again, perhaps mechanical bounce would be inevitable.

    Are either of these ideas correct, or is there something else I'm not seeing?
    Last edited by nak81783; 08-05-2013 at 10:31 AM. Reason: "Reactivity" replaced with "mechanical bounce" or "trigger reset" as necessary for desired or undesired trigger action.
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  2. #2
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    The ULT was designed to make the trigger lighter and remove reactivity at the same time.

    The smaller head diameter reduces the amount of force that the tank pressure can exert on the sear when the front chamber is empty. If the top of the pin was as large as the retro on-off pin, then reactivity would be increased using a smaller diameter pin in the lower section.

    When you pull the trigger of a retro valve, you are only pulling against the force exerted on the small diameter of the pin. The top and bottom of the larger head of the pin are at the same pressure so they almost cancel each other out and the only difference in the two surfaces is the small diameter of the pin. It is the only sealing surface when the chamber is pressurized and the direction is down onto the back of the sear. This is what gives the ULT such a light trigger pull.

    The amount of reactivity is dependent on force differential. The mass of the sear and trigger require a fixed amount of force to start its movement. In order for the pin to work reliably at standard tank pressures, the diameter of the pin top needed to be a certain size to provide that force.

    Shims couldn't stop reactivity on a ULT with a larger retro sized top head and smaller lower diameter without making the trigger pull abnormally long.
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  3. #3
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    Quote Originally Posted by athomas View Post
    The ULT was designed to make the trigger lighter and remove reactivity at the same time.

    nak81783: Remove reactivity, meaning mechanical bounce or hard trigger reset?

    The smaller head diameter reduces the amount of force that the tank pressure can exert on the sear when the front chamber is empty. If the top of the pin was as large as the retro on-off pin, then reactivity would be increased using a smaller diameter pin in the lower section.

    nak81783: How so? I think trigger reset force would actually be reduced slightly, because residual dump chamber pressure would act on a larger surface area. I am comparing std RT on/off pin with a hypothetical pin having std RT head diameter and ULT shaft diameter. I can't tell if you're comparing ULT to RT or hypothetical or some other combination.

    When you pull the trigger of a retro valve, you are only pulling against the force exerted on the small diameter of the pin. The top and bottom of the larger head of the pin are at the same pressure so they almost cancel each other out and the only difference in the two surfaces is the small diameter of the pin. It is the only sealing surface when the chamber is pressurized and the direction is down onto the back of the sear. This is what gives the ULT such a light trigger pull.

    nak81783: Understood and agreed.

    The amount of reactivity is dependent on force differential. The mass of the sear and trigger require a fixed amount of force to start its movement. In order for the pin to work reliably at standard tank pressures, the diameter of the pin top needed to be a certain size to provide that force.

    nak81783: Understood, but a larger pin head diameter would also produce that minimum required force.

    Shims couldn't stop reactivity on a ULT with a larger retro sized top head and smaller lower diameter without making the trigger pull abnormally long.

    nak81783: Why? As you stated, pressures on top and bottom of pin essentially cancel out. One is only pulling against the shaft diameter, regardless of the head diameter.
    I edited my original post. Reactivity can mean mechanical bounce or trigger reset. I realize they are related, but, in a tournament setting, mechanical bounce is bad, but a harder trigger reset is good. Hopefully, this clears up my question. I also commented on your response in the quote box.

    Do you know what different combinations of head and shaft diameters were tested?
    Last edited by nak81783; 08-05-2013 at 06:51 PM.

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    The reactivity I was referring to was the trigger reset, not mechanical bounce. Hard trigger reset is different than reactivity even though they are related. Reactivity in a retro valve is due to the difference between the force required for the trigger pull and the force required to reset the trigger. If there is a large differential in the force, your finger can't react fast enough to release the trigger when it is reset. The result is another trigger pull. It isn't mechanical bounce but it results in additional shots each time the trigger is pulled.

    The top of the large section of the pin is all that really matters when tank pressure is resetting the trigger. The bottom section of the pin is all that really matters when you are pulling the trigger. Its the difference between the two that creates reactivity in a retro valve.

    The larger size of the retro pin has more force than the smaller diameter of the ULT top. The bottom side has some force due to the residual pressure left in the chamber as well. The differential pressure would be ( ~850psi (top) minus ~100psi (bottom) = 750psi) pushing down on the top of the pin. The lower residual pressure in the chamber has no significant effect pushing back on the top of the pin, even if it doubled the contact area on the bottom side of the pin. Doubling the area at the same pressure would be the same as doubling the pressure with the same area so the new equation would be (850 - 200 = 650psi) acting on the top area of the pin.

    The larger diameter of the pin would have a much larger effect on the top force because of the much larger pressure exerted on the top. If the area only increased by 25% then following the same pressure area combination, ((850 x 1.25) - (100 x 1.25 x 2) = 812.5psi). Noticed I even doubled the bottom pressure to compensate for the diameter of the smaller ULT pin to give more of an advantage to the chamber pressure side pushing back. The result is that the return force increases dramatically by 25%. Now combine that with a lighter trigger force required to pull the smaller diameter pin and you have uncontrolled reactivity. Note: The pressures aren't actually going to increase 25% to 812.5 psi. This just illustrates the differences because the force is directly tied to pressure. The force differential will actually increase by that amount.

  5. #5
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    Quote Originally Posted by athomas View Post
    If there is a large differential in the force, your finger can't react fast enough to release the trigger when it is reset. The result is another trigger pull. It isn't mechanical bounce but it results in additional shots each time the trigger is pulled.
    This sure sounds like mechanical bounce to me, or, at least, most of it. I consider mechanical bounce to be the inability to prevent another shot because of the "elasticity" in a mechanical system. This can be from forces, vibrations, trigger slop, human finger elasticity, oring "sponginess", etc. What am I missing, or why is my definition incorrect?

    Please review this screenshot. The math shows what you were talking about. I used oring ID sizes, not actual pin diameters (too lazy to measure or look them up); it should be close enough for discussion purposes. If the reactivity couldn't be adjusted out with shims and/or a different top on/off oring (quad vs. urethane, etc.) for the RT-head/ULT-shaft scenario, surely it could be for an 005-head/001-shaft. It's almost identical to a standard RT.

    RT vs ULT.jpg

    Don't get me wrong. The ULT is a great product. I am just curious to its history as of late, so I wanted to discuss it. I was hoping to see what prototypes failed and why.

    Edit: Oh, for the love of Pete! How do I make my attached images larger without hosting?
    Last edited by nak81783; 08-05-2013 at 07:44 PM. Reason: Doh! Small image question.

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    Quote Originally Posted by nak81783 View Post
    This sure sounds like mechanical bounce to me, or, at least, most of it. I consider mechanical bounce to be the inability to prevent another shot because of the "elasticity" in a mechanical system. This can be from forces, vibrations, trigger slop, human finger elasticity, oring "sponginess", etc. What am I missing, or why is my definition incorrect?
    In the case of the retro valve its more hydraulic action than purely mechanical bounce. If you hold a stead force in a forward direction with little or no opposing force the object will move in that direction. If you then then apply a stronger force in the opposite direction the item being forced will travel in the opposite direction. When you remove the stronger force, the object will again revert to travelling in the forward direction. Its not mechanical bounce, but hydraulic action. You are holding a stead trigger force up on the pin causing the gun to fire and then applying a stronger force down on the pin to reset the trigger. Once the stronger action has completed and is no longer available, the upward trigger force is all that is remaining and the pin goes up again causing another shot to be fired. This will repeat until the cycle is broken by the trigger being held too tight or too loose.


    Quote Originally Posted by nak81783 View Post
    Please review this screenshot. The math shows what you were talking about. I used oring ID sizes, not actual pin diameters (too lazy to measure or look them up); it should be close enough for discussion purposes. If the reactivity couldn't be adjusted out with shims and/or a different top on/off oring (quad vs. urethane, etc.) for the RT-head/ULT-shaft scenario, surely it could be for an 005-head/001-shaft. It's almost identical to a standard RT.

    RT vs ULT.jpg

    Don't get me wrong. The ULT is a great product. I am just curious to its history as of late, so I wanted to discuss it. I was hoping to see what prototypes failed and why.

    Edit: Oh, for the love of Pete! How do I make my attached images larger without hosting?
    I'm not sure of the numbers used in ULT prototyping. I suspect the shaft size was the smallest that could be used without compromising the strength. The larger end was probably calculated based on the smallest size to meet the required force to reset the valve assembly without affecting the rate of fire.

    The reactivity can be adjusted out with shims (removal of shims actually). You have to start with a long pin (no shims) and then add shims to get the proper distance so that the on-off opens and closes properly with the sear. Removing shims makes the distance longer and therefore easier for a human to feel and interact with the movement. The problem is that most users want a short trigger pull, not a long one.

  7. #7
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    Ok, technically, I'll buy it's pneumatic bounce (gas), not hydraulic (liquid). Regardless, both are tangible means to provide mechanical forces, as opposed to electronic signals and magnetic fields which would be covered under electronic bounce.

    Good discussion. Thanks for the input.

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    Quote Originally Posted by nak81783 View Post
    Ok, technically, I'll buy it's pneumatic bounce (gas), not hydraulic (liquid). Regardless, both are tangible means to provide mechanical forces, as opposed to electronic signals and magnetic fields which would be covered under electronic bounce.

    Good discussion. Thanks for the input.
    The emag full auto would be an example of purely electronic bounce where the magnetic field is changed by the activation/deactivation of the solenoid. The interaction of the HES is fully electronic requiring no internal or external movement of parts. If you know the values of the parts, you could accurately predict the rate of interaction in this case.

    Other mechanical switches used in the triggers of many markers provide bounce using mechanical means to manipulate an electronic signal. It is often referred to as electronic bounce, but it is purely mechanical in the way it works. The action of the switch actually causes the contact to physically bounce off the contact resulting in a make/break/make.... scenario until the force of the spring in the switch prevents the contact from leaving the contact, in which case it is fully closed and conducting. This relies on the human finger and is harder to accurately predict the rate of interaction but can be tuned out using electronic filters.

    The pneumatic interaction of the retro on-off or ULT is very predictable and is actually designed to operate that way. Knowing the pressures being used, you can accurately tune the amount of reactive force you feel and use to operate the trigger mechanism. It is the only really tune-able system. Even the other electronic systems rely on accurate mechanical systems in order to work properly.

  9. #9
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    That makes sense.

    I did get you to say pneumatic, though.

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