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tmc_31
02-27-2015, 12:33 PM
Hey guys,

I want to build a device that will go in front of a hockey goal to block part of the opening of the goal. I know that the specs for a hockey goal require the front frame to be built from 2-3/8" schedule 40 tubing. The device would consist of a square steel tube frame that would have a net laced into it. I am trying to determine is what size square steel tube I need to use for this thing.

I am not an engineer by any stretch of the imagination. My normal approach to this problem would be to build it out of 2X2X1/4" square steel tubing and hope for the best. However, I need to build it as light as possible and still not have it deform under the stress of a slap shot.

What I know:

1) A regulation hockey puck weighs 6 ounces, some practice pucks weigh up to 10 oz.

2) The speed of the puck during a slap shot can be as high as 108 mph.

3) Human life will not be endangered if the device fails.

What I would like to know:

1) What force (in psi) will the puck impose on the steel frame if struck head on?

2) Are there other forces at play besides a direct strike from the puck that would contribute to deformation?

3) What would be an appropriate safety factor to use in this instance?

3) How do I apply this information to determine what size (structural) square steel tube to use?

Thanks,

Tim

Toolguy
02-27-2015, 12:54 PM
Will this part have an opening in the middle or be solid all the way across?

cameron
02-27-2015, 12:54 PM
Any concern about a player sustaining a more serious injury slamming into the corner of the square HSS, as compared to the 2 3/8 " round?

Royldean
02-27-2015, 01:07 PM
I believe that the force will be way too low to damage 2x2x.25 steel tubing, but really it's impossible to calculate without knowing some very specific properties of the puck (due to it's material properties and shape). Short of some skilled FEA (where getting the material properties of the puck will be the most complicated part), there's no easy way to predict.

But I estimated that if 95% of the energy from the strike is absorbed into the steel, you are only looking at an average force of 70lbs.... but again, due to the geometry of the puck, and the material properties, the impulse time and area seeing the actual strike could vary widely. 70lbs is a lot of force when concentrated on a small area.....

tmc_31
02-27-2015, 01:13 PM
Toolguy, it will have a net stretched across the front of it. The frame is only there to support the net.

Good point cameron but that hazard exists whether or not the frame breaks or deforms. Since this is used as a training device where only one person is using it at a time I think the increased hazard is minimal. Still I will do well to grind off any sharp corners at the weld points. The edges of structural square tubing are pretty well rounded anyway.

Tim

brian Rupnow
02-27-2015, 01:34 PM
Make it out of 1 1/4" x 1 1/4" x 0.100" wall mild steel square tubing.---Brian

Black Forest
02-27-2015, 02:03 PM
Whar are the dimensions of the square and how will it be supported?

tmc_31
02-27-2015, 03:27 PM
Thanks for the responses guys,

Royldean, I think you are right, 2X2X.25" should certainly be sufficient. However I am trying to come up with the smallest/lightest square tubing that will withstand that force. How did you come up with the 70 lb figure? Am I correct in assuming that would be 70psi?

Brian, thank you. Would you mind sharing how you came up with that? (keep in mind that this is for my and others education as much as for coming up with the right size tubing).

BF, frame is an irregular shape that fills probably 65% of the opening of a hockey goal (72"X48"). The longest tube that is supported at each end is the 72" bar across the bottom.

Tim

Paul Alciatore
02-27-2015, 03:40 PM
I don't think that average force is the way to go here. When the hockey puck strikes it, there will be a very high amount of force over a very small area when it first makes contact. Then this force will decrease as the puck or the steel distorts to allow the contact to spread over a larger area. But the damage will have already been done by the high initial force. If you want to prevent any damage to the frame, you are going to have to make it stronger than the average force calculation will indicate.

I would look at some kind of covering or coating that will be tough enough to hold up but that has enough "give" to take up that high initial force. That covering would also make it more user friendly when someone runs into it.

The easiest way to do this is probably experimentally. Set up a test section and hit it with high velocity hockey pucks.




I believe that the force will be way too low to damage 2x2x.25 steel tubing, but really it's impossible to calculate without knowing some very specific properties of the puck (due to it's material properties and shape). Short of some skilled FEA (where getting the material properties of the puck will be the most complicated part), there's no easy way to predict.

But I estimated that if 95% of the energy from the strike is absorbed into the steel, you are only looking at an average force of 70lbs.... but again, due to the geometry of the puck, and the material properties, the impulse time and area seeing the actual strike could vary widely. 70lbs is a lot of force when concentrated on a small area.....

tmc_31
02-27-2015, 04:02 PM
Ha, Paul, I know just the guy that can test this for me:). My son is a hockey player and he asked me to build this for him. Why should I have all the fun, He has been a fan for many years and has just started playing in an adult league a year ago or so. His son (my 6 yro grandson) also has been playing for 2 years. It is a lot of fun to watch those little guys (and gals) go:).

That piece of tubing is going to temporarily deform some during the strike. The puck will also deform some absorbing some of the energy. The trick is going be to find out how large the tubing member will have to be to not deform permanently under the forces involved.

I am thinking this is a problem that could be solved as a beam stress calculation with a 72" beam supported on each end and the force applied at the center of the beam. I am not sure how the beam would react with a strike (quick impact) as opposed to a slowly applied force.

For the sake of argument, I am assuming that the impact will be spread out over about 1/2" due to the deflection of the puck.

Tim

brian Rupnow
02-27-2015, 04:09 PM
That size I gave is not one arrived at by engineering calculation. It is based on 50 years design and fabrication experience. 1" square tubing will work, but it will bend---not from anything a puck will do, but from bodies running into it. 1 1/2" will work fine, but it is heavier than you really need. As people have pointed out, any hit from a puck on the network suspended from the square tubing will evenly distribute the shock load over a large area and have no effect on the square tubing. A hit directly on the frame by a slap-shot puck isn't going to damage it, because there is enough "give" in the structure that it will briefly deform and mitigate the "point load force" of the puck on the tubing.

tmc_31
02-27-2015, 08:14 PM
Thanks Brian,

Given your background and barring any better information forthcoming, I will go with 1-1/4" .100 wall square tubing. Thanks for your input!

Tim

brian Rupnow
02-27-2015, 08:32 PM
Post a picture when you get it finished. Good luck!!!---Brian

Royldean
03-02-2015, 08:58 AM
Am I correct in assuming that would be 70psi?


No, 70lb would be the average sustained load during the impulse. The stress would be that load multiplied by some factor (such as the inverse of the cross sectional area if the loading were pure tension or compression, etc). But loading in this case would be quite unique and difficult to calculate by hand based on the cross section of the tubing and the form of the structure... but FEA makes that part really easy.

rowbare
03-02-2015, 10:43 AM
Sounds complicated. I would have thought that a sheet of 3/4" plywood and half a dozen conduit clamps would be all it takes...

bob

tmc_31
03-02-2015, 10:57 AM
Thanks Royldean,

Back to the drawing board:)

Unfortunately, I don't have FEA software available to me, I don't know how to run it and if I did, I doubt if I could interpret the results correctly anyway.

I had hoped that I could treat the longest member (72") as a beam pinned at each end and the load applied at the middle and use something like beam boy or webstructural to determine the deflection.

I played with webstructural (webstructural.com) some and the results showed that I would be safe with a 2X2X.125" square steel tube. Unfortunately, that is the smallest square steel tube in it's database. I had hoped to analyze the 1.25X1.25X.125" tube that Brian suggested in an earlier post.

Tim

oldjim
03-02-2015, 06:57 PM
I have been retired for 20 years, so some of you active engineers can correct me as needed. This is dynamic problem I believe not a statics problem. I lot of you guys will remember the very basic formula F=MA, where F = Force (what you are looking for)= the Mass of the puck, times A, and Of course in this case the A we need is the deceleration rate of the puck.

So therefore the stiffer you make the "frame", the shorter you make the deceleration time and thus the greater the force. The fact that the hockey puck will deform on impact increases the deceleration time and reduces the force somewhat. Think what would happen in the problem if you replaced the puck with a steel block of the same weight!! Although the weight would be the same the steel block would have close to no give and the force would increase measurably.

One thing to keep in mind is the force on the frame has to be resolved somewhere (foundation bolts ?) With this in mind, the frame of smaller tubing (of course of the right size) would work well. Because it will give a little and thus increase deceleration time. Of course in the real world you could make it, as we use to say, "HEAVE DUTY STEEL MILL CONSTRUCTION) and not worry about it. However as you worded your request, I had the feeling you were looking to make something a bit nicer.

I might suggest, take the problem just as you have stated it, to your local college and show it to the Statics and Dynamics prof. Teachers loved to get these "real world" problems and give to their students.

tmc_31
03-02-2015, 08:28 PM
Thanks oldjim,

We have 3 colleges here but not an engineering program in the bunch:(

I will do a little looking though

Tim

RussZHC
03-02-2015, 09:26 PM
I would view this as an opportunity to makes lots of neat stuff :cool:

Testing is the way to go to accomplish that...find yourself something like a skeet/clay pigeon machine, or build one, so you can fire pucks all day long at hunks of metal supported in different ways.
Guessing one of the first things you will find out is that round tube has a bunch of advantages vis a vis the forces since except for that small percentage of blows, most of the impacts will "deflect" to some degree (as opposed to a face square to all impacts).

Based on your description, I assume you want to accomplish what this does http://www.hockeytrain.com/the-ultimate-goalie.html but sort of in "reverse" where what you build will leave portions of the goal open, aiming to put the puck in those spaces?

PStechPaul
03-02-2015, 09:53 PM
This problem brings to mind the forces that certain animals can exert with their bite, and is often used as a way of making people afraid of pit bulls. But I often see it expressed as PSI, but that is more a function of the sharpness of the tooth that is exerting the force, and the deformation of the tooth and the object being bitten. A wolf can have a bite pressure of over 400 pounds, while domestic canines are generally under 350. Guess which animal tops the chart and what the pressure is:

http://channel.nationalgeographic.com/wild/dangerous-encounters/videos/wolf-bite-force/

Here is a list which (incorrectly) states the force in PSI:

http://forums.sherdog.com/forums/f48/top-20-worlds-strongest-animal-bite-forces-new-2393047/

I think it should be possible to determine the effect of a hockey puck hitting a piece of steel tubing by doing a test similar to those which measure hardness and elasticity. It may be necessary to make a WAG as to the surface area of contact at maximum deformation of the puck and the steel tube, but at worst it is probably no smaller than 0.1" square or 0.01 square inches. The force would be the mass times the deceleration, which would be the change in speed (maximum to zero), and if the deflection is about 0.1", the time would be 0.1"/0.5(100 MPH) = 0.1/0.5*880 IPS or 0.22 mSec. Deceleration is 880/0.00022sec = 320,000 F/sec/sec, which is about 21,000 G. so the force exerted for a 1 pound puck would be 21,000 pounds or 10 tons. I may be wrong with my math, but I think it is reasonable.

[edit] I had an idea that the sound of the hit may reveal the time of deceleration, and 0.22 mSec would be a period of 0.44 mSec and a frequency of 2.27 kHz.

Now to determine the effect on a piece of the tube, you could put it in a press and deflect it by the estimated 0.1" deflection (reading on a dial indicator). You may also measure the pressure required to do this, and it will certainly be much less than 10 tons. Then release the pressure, and if it returns to the same original position (zero on the dial), the elasticity is sufficient and no permanent deformation has occurred. You may also try increasing amounts of pressure to determine the point where it does not return to zero.

You might also use a piece of 0.125" drill rod with about 0.012 square inches area and press that onto the tube to see if it makes a dent. I doubt that the puck would be capable of doing so, and it probably would not do any more than cosmetic damage.

You may be able to simulate the puck hitting the tube by fastening a puck to a weight and drop it from a known height so that the energy is the same as a 1 pound puck traveling 100 MPH. One G is about 15 feet/sec/sec. so a 6 foot (2 meter) drop should reach 22.5 km/hr, or 14 MPH. So since energy = m*v^2, 100 MPH would require 100^2/14^2 = 50 times the mass. The dynamics are different at higher speeds because of the inertia of the structure, but this might provide some idea of what might happen. If you use a high speed camera you might see how far the tube deflects, or you could put something like soft putty under the tube and measure how far it squahes.

Oops - I think I got a bit carried away... :rolleyes:

tmc_31
03-04-2015, 01:55 AM
Thanks Paul,

Lots to think about there. It will take me a bit to work through the math. Interesting thought using sound to measure the duration of the strike:)

Know where I can borrow a gator for a few days?

Tim

Lee Cordochorea
03-04-2015, 11:39 PM
I'm assuming the "regulation" uses nominal 2" schedule 40, which has an actual external diameter of 2.38" and a wall thickness of 0.15"

So, if we start with regulation pipe as "good enough," what size square tube will have the same umph? ("Umph" is a technical term which may or may not mean "section modulus" or some other fancy set of words.)

tmc_31
03-05-2015, 09:53 AM
Lee, you are correct about the "regulation" tubing. Since this attaches to the front face of the goal (zip tied in place), I am hoping to get away using some smaller (lighter) tubing. I am going to try the 1-1/4" square tubing with a .120" wall and see how well it will withstand the abuse.

If it doesn't work, no harm no foul, I will just go back to the regulation material.

Tim

Rosco-P
03-05-2015, 10:07 AM
You seem to be designing this around square tubing. Any reason why that choice of material? Round tubing might tend to deflect the puck and absorb less of the energy from the shot.

Dave P.
03-05-2015, 10:56 AM
Having been hit by about 27,000 pucks over the last 50 years, I'd say just go buy some cheap black steel "water pipe"
maybe 1in dia. and don't worry about it.
Sometimes people over think solutions......
Dave

peekaboobus
03-05-2015, 01:56 PM
You seem to be designing this around square tubing. Any reason why that choice of material? Round tubing might tend to deflect the puck and absorb less of the energy from the shot.

why would round tubing absorb less energy?

becksmachine
03-05-2015, 03:54 PM
why would round tubing absorb less energy?

Maybe because there would be less likelihood of being struck on an axis that would result in no sideways deflection of the puck.

In other words the axis of diameter (?) of the round pipe exactly coincident (collinear?) to the direction of force, which would force the pipe to absorb 100% of the kinetic energy of the puck.

Dave