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To do some brain workout, I calculated 3-nuts's design.

naidanac(劳者)

This is a beam supported at both ends with a single center load. The most concern in this appliaction is the compressive stress along the beam span.

The section of a 2x4 lumber is 3.5" x 1.5", beam length 10 FT,

Maxium compress stength of grade 2 white pine is 4800 psi, from my calculation, allowed max load at the beam center is about 490 lbs. With double 2x4 lumber, the max allowed load is 980 lbs.

Take safe factor as 3, max load for this design should be 330 lbs. Under such load, the sag at the center is about 0.9 inch.

Shear stress under 330 lbs is 63psi, which is far below the max shear strength 900 psi.

In conclution, I would appove this design if I have a stamp.

(#16368386@0)
Last Updated: 00:37

Replies, comments and Discussions:

枫下家园 / 美丽家居 / 简易木制龙门架的制作 -初中物理学课本知识的直接应用 -ysysning(三果); 11-1 21:40 {2447} (#16368141@0) +2
人类早期历史上简单的发明之中，轮子和杠杆是对生产力的进步作用是最大的两大工具发明。

因为有时候要运输大件货物，皮卡车的后盖有时候也要取下来，120公斤又大又沉，两个人都费力。有前几个月手动搬了一次，感觉腰肌劳损，当时决定再也不出劳力了。如何建造合适的工具器材成为迫切需要。有商品的铁制龙门架最便宜的也要两三千加币，而且太沉根本无法作为住宅使用，我只需要两三百磅的承受力，不需要一吨的。网上找人估价铁杆焊制报价六百也觉得太贵。本打算不再diy的，忍不住又开工，电动绞盘150$ ，木材及其他小配件50$,设计应力 500磅。 2“x"4x10'水平放置中间压力可承受300lbs 两根600lbs,高于设计要求。

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Cool! I have a question. How do you calculate the max load? -sailor(Ocean & Mountain); 11-1 21:51 (#16368175@0)

这不是计算题。第一需要知道max load, 第二，需要实验知道lumber能够承受多少。 -rustalk(Sniper); 11-1 21:55 (#16368179@0)
@all, you didn't read the full post yet before asking this question. -ysysning(三果); 11-1 21:55 (#16368184@0)

You said one beam is good for 300lb. How did you know it? -sailor(Ocean & Mountain); 11-1 21:59 (#16368190@0)

商店的木材都是有压力和剪切力的应力标准的，这是房屋材料的产品质量要求，数据网上可以查到，不需要自己测试。另外我大学的毕业设计是上海郊区的一座三跨小桥当然是抄袭前人图纸的，鸭嘴笔描的。 -ysysning(三果); 11-1 22:01 (#16368195@0)

You deserve to be rich. So knowledgeable! 😁 -sailor(Ocean & Mountain); 11-1 22:04 (#16368207@0)

To do some brain workout, I calculated 3-nuts's design. -naidanac(劳者); 00:37 {713} (#16368386@0) +1
This is a beam supported at both ends with a single center load. The most concern in this appliaction is the compressive stress along the beam span.
The section of a 2x4 lumber is 3.5" x 1.5", beam length 10 FT,
Maxium compress stength of grade 2 white pine is 4800 psi, from my calculation, allowed max load at the beam center is about 490 lbs. With double 2x4 lumber, the max allowed load is 980 lbs.
Take safe factor as 3, max load for this design should be 330 lbs. Under such load, the sag at the center is about 0.9 inch.
Shear stress under 330 lbs is 63psi, which is far below the max shear strength 900 psi.
In conclution, I would appove this design if I have a stamp.

👍👍👍 -sailor(Ocean & Mountain); 00:40 (#16368390@0)

不错像是微软游戏帝国时代中的投石机就差大法师念咒作法了😂👍 -eleclan(eleclan); 11-1 22:06 (#16368215@0) +1

还真是啊多年前的游戏了就差块石头了 -ysysning(三果); 11-1 22:37 (#16368270@0)

Aliexpress 上有很多江浙乡镇企业做的类似的工具。小几千块钱，就能搞定。 -georgeh1(George); 11-1 22:41 (#16368279@0)

研究过， -ysysning(三果); 11-1 22:51 {375} (#16368297@0)
很多设备，有的小型升降机的一两百磅重根本没办法搬运，我又在准备另一种方案，尺寸不符合我需要，我需要加长加高，承重六百磅，是猎鹿人把猎物抬到皮卡车里的架子。

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三果哥是我大中华第一土木名校毕业的，作品虽然简陋，但是你们不能怀疑用到的知识的准确性 -hgej(PEO); 11-1 22:45 (#16368284@0) +1

惭愧这个专业已经招不满学生了 -ysysning(三果); 11-1 22:52 (#16368300@0)

不是招不满，而是很多学校干脆停办这个专业了，起码是暂停招收本科生，不过应该没人灰度这个专业的研究生 -eleclan(eleclan); 11-1 22:55 (#16368304@0)

搞那么复杂，邻居没已经？看你家路边不是有树吗，绑个pulley拉拉不就行了？ -hct01(Hct01); 11-1 22:47 (#16368288@0)

可快速拆卸得平时放后院想过树但是车不能开到草坪上树枝不解释一两百磅枝干肯定承受不了 -ysysning(三果); 11-1 22:54 (#16368303@0)

200磅一个人爬树枝能断吗？ -hct01(Hct01); 11-1 22:57 (#16368305@0)

我家前面得树除了主干旁支肯定能 -ysysning(三果); 11-1 22:58 (#16368307@0)

@San Francisco

To do some brain workout, I calculated 3-nuts's design.

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