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Slap it or pull it?

Post questions and info about pneumatic (compressed gas) powered cannons here. This includes discussion about valves, pipe types, compressors, alternate gas setups, and anything else relevant.
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Slap it or pull it?

Unread postAuthor: boyntonstu » Wed May 12, 2010 6:51 am

Using a spring or a sligshot rubber, would the ball valve open quicker if the spring was triggered and immediately began pulling on the handle, or would it be quicker to accelerate a mass and slap the handle open?

(Pitching vs hitting a baseball with a bat)
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Unread postAuthor: jrrdw » Wed May 12, 2010 7:13 am

In most situations that I know of it is all ways easier to pull then to push. A straightly lined mechanism will help greatly...
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Unread postAuthor: boyntonstu » Wed May 12, 2010 8:09 am

jrrdw wrote:In most situations that I know of it is all ways easier to pull then to push. A straightly lined mechanism will help greatly...



Is a 4 second 1/4 mile dragster pulled or pushed?

It would be nice to find out if a pushed train is more or less efficient than a pulled train.

BTW FWD cars are not good pullers going uphill because the weight is shifted to the rear.
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Unread postAuthor: saefroch » Wed May 12, 2010 9:48 am

I would say it would be more effective to accelerate a mass to push on the handle. The only issue there is the question of an effective and reliable mechanism, but we would really need more details... even though it seems you have already decided. A dragster cannot really be considered pulled or pushed. For trains it ought not to make any difference. And yes, front-wheel drive cars are not as good at going uphill, but only because of the normal force change, which has nothing to do with the question of pulling or pushing.
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Unread postAuthor: Ragnarok » Wed May 12, 2010 10:08 am

boyntonstu wrote:Is a 4 second 1/4 mile dragster pulled or pushed?

He said easier. Not necessarily faster. The problem with such a dragster is that it needs fairly impressive design to simply keep it on the ground and going in a straight line.

In this case, a spring loaded mechanism that pulls directly on the ball valve will probably be slower - however, it is a less complicated system. Building some kind of "hammer" to knock a ball valve open will take a larger system that needs to be built stronger.

In short, a system will self stabilise if the accelerative force is applied on the same side of the centre of gravity as the direction it's trying to accelerate the mass in.

It would be nice to find out if a pushed train is more or less efficient than a pulled train.

In this case, irrelevant. Either way, the rails are there to guide the train. Any differences would be aerodynamic.

FWD cars are not good pullers going uphill because the weight is shifted to the rear.

Ah, now cars are an example that makes jrrdw's point quite well.

Take a FWD and a RWD car.

With the FWD car, the accelerative force is applied to the road ahead of the centre of gravity. And that means you can get power understeer - which is the car self-stabilising (i.e. continuing to try and move in a straight line)

With the RWD car, the accelerative force is applied to the road behind the centre of gravity. And that means you can get power oversteer - which is the car destabilising (i.e. spinning out.)

Basically, to draw a parallel to projectiles, an FWD car is like putting the centre of pressure behind* the centre of mass.
A RWD car is like putting the centre of pressure ahead* of the centre of mass, an arrangement which will naturally destabilise unless something else forces it to be stable (e.g. gyroscopic effects, or the driver)

*These are "the other way around" because drag is a deceleration force, not an acceleration one like a car engine.
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Unread postAuthor: Technician1002 » Wed May 12, 2010 10:11 am

Hit a dragster with a train traveling 200 MPH and then consider the quarter mile time. You do have to consider the forces in your sudden impact and acceleration when striking something to accelerate it.

My valves are opened by an impact followed by heavy acceleration from air pressure. It gets the slow starts fixed. :D The trigger rod has mass and free travel distance before it engages the piston. It works best with a sharp pull.
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Unread postAuthor: jrrdw » Wed May 12, 2010 10:47 am

FWD vs RWD is a bad example simply because the mechanism differences control the outcome.

Example: build a FWD with the engine in the rear with the drive shaft pointing to the front with driven axles as the standard RWD is designed. In this example would it be easier to move it because it's being pulled? Or would it be easier to move said car because of the design?

Less parts are easier to move, less mas/weight/friction etc...

I think the train example is bad as well. Look at the 'Mag-lift'. Is it pushed or pulled?

By the way trains have been designed to be pulled every since they have been made. Not until the build of the diesel locomotive have they been made to push, and there is normally a engine in the front when that is going on.

There are to many varying factors to say one way or another. One way can be made to work easier then the other when it comes to a ball valve. :D
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Unread postAuthor: boyntonstu » Wed May 12, 2010 11:09 am

jrrdw wrote:
By the way trains have been designed to be pulled every since they have been made.


With all that wood smoke, one would have imagined pushing instead.
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Unread postAuthor: jrrdw » Wed May 12, 2010 11:23 am

boyntonstu wrote:
jrrdw wrote:
By the way trains have been designed to be pulled every since they have been made.


With all that wood smoke, one would have imagined pushing instead.


It's up top out of sight unless looked for, not a concern until environmental conciseness took hold...
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Unread postAuthor: Technician1002 » Wed May 12, 2010 11:42 am

The reason trains are pulled has to deal with wheel alignment. Pushing a train make the train zig zag on the track as far as the rails permit. Pulling the train pulls all the wheels in line. It also puts the engineer where they can see things ahead.

Back on subject with another example.. Golf. Place the club right behind the ball and swing, vs swing and hit the ball. Which shot with the same force of arm power gives better distance and faster acceleration of the ball?

Same applies to a spring loaded mass with speed before reaching the valve. It starts to open later as you have to wait for it to make contact, but after contact, the valve speed in getting open is faster.


Again, this is used to eliminate slow startup of my valve. The golf ball, rod and hand picks up speed before contact with the valve piston is made. After contact the piston picks up speed rapidly from being struck.
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QDV sliding hammer opens the valve.
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Unread postAuthor: boyntonstu » Wed May 12, 2010 12:11 pm

Technician1002 wrote:The reason trains are pulled has to deal with wheel alignment. Pushing a train make the train zig zag on the track as far as the rails permit. Pulling the train pulls all the wheels in line. It also puts the engineer where they can see things ahead.



Ships are propelled from the rear screws with lookouts up front.

A string to the rear and a bell would perhaps keep the wood engine engineer RR alive (forgetting Mr. Zig and Mrs. Zag).

Thanks for explaining wheel alignment and zig zag.

Using my rill press I was able to reduce the force from 10 pounds to 2 pounds measure at the same torque position.

In addition, the ball valve is closed at about 80 degrees without leaking.

If I can equal or surpass my Trom-Boyn piston valve performance with the crossbow design, it would be a very easy to build and inexpensive rifle.

A slap shot is something to consider.
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Unread postAuthor: jrrdw » Wed May 12, 2010 12:27 pm

A slap shot is something to consider.


Have you thought about damage from getting hit over and over? Will the force of a 'slap shot' drive the lever to the end of it's travel?
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Unread postAuthor: POLAND_SPUD » Wed May 12, 2010 1:12 pm

let's start off by stating that ball valves suck... linear movement is much better than rotations as far as opening speed is concerned, force needed and complexity

the only reason why people use ball valves is becasue they are cheap and inexpensive....

you'll get better performance with the QDV valve... and yeah you can use a springloaded hammer to actuate it too
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Last edited by POLAND_SPUD on Wed May 12, 2010 2:05 pm, edited 1 time in total.
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Unread postAuthor: Demon » Wed May 12, 2010 1:39 pm

I always wondered if the way that the ball valve opens and the air flows has any influence on the accuracy

Compared to my piston cannon, my spudgun with a ball valve made the potato curve 80 % of the time in a random direction with the same barrel
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Unread postAuthor: Gun Freak » Wed May 12, 2010 3:49 pm

There is a design in boat motors that uses a propellor under the bow and pulls the boat along, supposedly being more efficient.
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