what is meant by Clearance from Contact Depth Line

Nandyk
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what is meant by Clearance from Contact Depth Line

Postby Nandyk » Sat Dec 22, 2012 8:50 pm

Hi,

My first time to Boomilever. I have 3 questions.
1. I was going thru the scoring sheet and 6d in the sheet says "Clearance from Contact Depth Line (in cm to the closest 0.1cm)" what is this and how is this measured?
2. If I plan to have 2 holes in my base, how apart should they be? Any idea?
3. When I see the pictures of different boomilevers, I see different kinds of loading blocks. So Am I expected to go with my own loading blocks or the respective contests will have the loading blocks in the size of 5x5x2?

Many thanks,
NandyK

retired1
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Re: what is meant by Clearance from Contact Depth Line

Postby retired1 » Sat Dec 22, 2012 9:04 pm

Reread the rules. The distance is from the center of the bolt hole on the wall to a line 20.0cm below that.Your boom has to be no lower than the line where it touches the wall. I would not recommend building any of the rest lower than that. An elliptical boom would be an exception, but that is not for a beginner.
If by 2 holes in your base you are talking about your boom attachment point to the wall, then theholes have to be the same distance as the rules give you for the wall construction.
Again, the rules say that the loading block will be provided by the event supervisor and as close to 5 x 5 x 2 as they can make it with a hole in the center for the eye bolt and a chain with s hooks.

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Re: what is meant by Clearance from Contact Depth Line

Postby Balsa Man » Sun Dec 23, 2012 3:46 pm

Nandyk wrote:Hi,

My first time to Boomilever. I have 3 questions.
1. I was going thru the scoring sheet and 6d in the sheet says "Clearance from Contact Depth Line (in cm to the closest 0.1cm)" what is this and how is this measured?
2. If I plan to have 2 holes in my base, how apart should they be? Any idea?
3. When I see the pictures of different boomilevers, I see different kinds of loading blocks. So Am I expected to go with my own loading blocks or the respective contests will have the loading blocks in the size of 5x5x2?

Many thanks,
NandyK


A couple quick additional thoughts/things to consider.
1) Since scoring is on efficiency- you want the lightest structure that can carry the load - you don't want extra wood that is doing nothing structural. Having something at the bolts- a piece that you can attach your tension members to is necessary. But wood in between the holes does nothing (but add weight, and lower your score). So think about one piece at each of two bolts - something that the bolt goes through, and holds in place, and that provides something to glue/attach the tension members to. This joint, as you will find- as a beginner -is a challenge; there is a lot of force on it.
2) Take the time to read through the message threads here in the boom section- go back to the beginning, read through, follow links provided, look at the photo gallery; the study portion of this building event. Understand how the forces in a boom work, and the options you have for controlling/mastering them.
3) regarding the contact depth line. Where it is is the distance below the line of bolt holes provided in the wall on which you hang your boom (for B-division, 20cm below)- it's a line parallel to the bolt hole line. What it is is the line below which your boom cannot touch the wall. As you will see when you do your homework, you want where the compression member(s) of your boom touch the wall to be very close to that line- you want as much height/distance between the top attachment to the bolts, and the bottom contact of the boom arm with the wall as you can. The shorter that distance is, the more the forces on the members of the boom. So you want a design/layout that gets you close to that line
Len Joeris
Fort Collins, CO

rjm
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Re: what is meant by Clearance from Contact Depth Line

Postby rjm » Sun Dec 23, 2012 7:55 pm

Here's a comment about that "Contact Depth Line". The rules as adopted are virtually a repeat of the 2007/2008 rules. As the rules were being written last Spring, there was interest in developing additional challenge, and also in capturing an additional measurement to be used in tie-breaking. These structure events have generally not had a good second tie-breaker. Building a Boomilever with a shallower wall contact profile is more difficult and demanding of a light structure than one which is deeper, and so the thought was to reward those shallower structures with a higher score, that may offset the probable increase in the mass of the structure. It would have been a multiplier applied to the efficiency score. How do you easily determine the depth of the boomilever on the wall? One way is to measure up from the 20 cm or 15 cm rule line to the lowest contact point of each Boomilever when it is mounted and ready for testing. There were concerns that judges would have a hard time doing this fairly and correctly (?), and so making it part of the primary scoring was dropped in favor of using it as a tie breaker only. Maybe this element will be added in the future if things go well this year, time will tell. Maybe we can get some feedback from teams and judges about whether a more complicated challenge for next year is a good idea.

Since the contact depth is only a tie breaker, and since ties are uncommon in this event, I will be advising my students not to worry about it.

Bob Monetza
Grand Haven, MI

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Re: what is meant by Clearance from Contact Depth Line

Postby nTvarify » Wed Jan 02, 2013 5:51 pm

So are you allowed in division C to build lower than 15cm from the base connection? As long as you only contact the wall <15cm from the base. Because for fun I want to try and build a triangular or elliptical compression member. If not clear I can try and draw a picture.

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Re: what is meant by Clearance from Contact Depth Line

Postby Balsa Man » Wed Jan 02, 2013 6:04 pm

nTvarify wrote:So are you allowed in division C to build lower than 15cm from the base connection? As long as you only contact the wall <15cm from the base. Because for fun I want to try and build a triangular or elliptical compression member. If not clear I can try and draw a picture.


I think your question is can pieces/part of the compression member be below the plane of the contact depth line - so long as contact with the wall is above that line. Yes, it is a "contact" line- it constrains how far down you can contact, not how far down, out away from the wall, parts of the structure can extend.- applies to both C- and B-
Len Joeris
Fort Collins, CO

rjm
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Re: what is meant by Clearance from Contact Depth Line

Postby rjm » Fri Jan 11, 2013 2:07 am

Len is correct, portions of the structure can be lower than the contact line for whichever division you are in. It just can't touch the wall lower. So, if you come up with geometry for your boom which extends out and down, that's okay. There is an ultimate limit, the loading block may not start out lower than 25 cm lower than the mounting holes. It will be hard for judges to tell unless they establish a reference line near the center of the tester. The only reason for such a rule is practical: Test stands may not be able to accomodate very low initial heights. Boomilevers sag dramatically, and sagging lower than 25 cm is within the rules. Low starting heights may be difficult and if your bucket touches the floor due to deflection, you're done testing, so keep that in mind.

Bob Monetza
Grand Haven, MI


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