Basic Machines CK draft
Basic Machines intro
The basic machines get used every day on your jobsite. If you understand the principles, you’ll be in a better position to solve problems that develop. Think of a simple but treacherous example. A building partially collapses and traps a worker under a big chunk of concrete floor. A back hoe can’t get into the space and you have to act quickly.
The piece of floor is about 8’ x 4’ and 6” thick. You know, or should know, that concrete weighs 150 pounds per cubic foot and that 8 x 4 x .5 = 16 cf. So 16 cf x 150 lb/cf = 2,400 pounds.
To lift the concrete off the trapped worker, you’d need 12 guys each lifting 100 pounds to lift half the weight, to lift it up like you were lifting the load on a wheel barrow (which is a 2nd class lever). But you don’t have 12 guys, you,ve got two guys and some long sections of 3” sprinkler piping. You know you could use the pipe as a 1st class lever to lift the slab.
So how long a piece of pipe do you need? If you’ve only got two guys to help with the lever, and one to pull the trapped worker out, you could assume that each guy could pull down 150 pounds, so you have a force of 300 pounds to work with. If you have a pivot point 2’ from your lift point, you need at lease 8’ of pipe on the side of pivot point to lift the 1200 pound concrete and free the worker. So if you grab a 12’ section of pipe, you free the trapped worker.
If you only have one person to lift, you probably could still free the guy if you have a 20’ section of pipe. The ability to do those quick calculations, and even more importantly, to understand the basic lever principles will be useful for the rest of your life. So why not take a few minutes and learn the basics of the 6 simple machines?
Reading Blueprints and Drawing Basic Sketches
I remember hearing in high school physics class that the language of science is mathematics. Since I never spoke that language too well, I wasn’t much of a scientist. I’ve been an effective contractor, though, and have learned the language of construction: drawings. We communicate in job trailers by pointing to the significant sections of drawings. We make sketches, often quite rough, to show what we’re trying to say.
In short, if you want to advance in construction, learn to read drawings well and to make rough sketches. It’s a simple language to learn (I’m the least visual person I know and I learned it), but it does take some studying.
I plan to review NAVEDTRA 14037 and write a simplified version to post directly on the site. For more detail the interested reader can come directly to this document. I will do a similar condensing of math sections (very condensed), first aid, safety, photography, blueprint reading, basic tools, fluid power, etc.
There are only six simple machines: the lever, the
block, the wheel and axle, the inclined plane, the screw,
and the gear.
Levers
You will find that all levers have three basic parts:
the fulcrum (F), a force or effort (E), and a resistance
R). Look at the lever in figure 1-1. You see the pivotal
point (fulcrum) (F); the effort (E), which is applied at
distance (A) from the fulcrum; and a resistance (R),
which acts at a distance (a) from the fulcrum. Distances
A and a are the arms of the lever.
Figure 1-1.-A simple lever.
#1 the mechanical advantage equals the length of the lever. So the mechanical advantage in Figure 1-12 is 5.
SUMMARY
Now for a brief summary of levers:
Levers are machines because they help you to do
your work. They help by changing the size,
direction, or speed of the force you apply.
There are three classes of levers. They differ
primarily in the relative points where effort is
applied, where the resistance is overcome, and
where the fulcrum is located.
First-class levers have the effort and the resistance
on opposite sides of the fulcrum, and effort and
resistance move in opposite directions.
Second-class levers have the effort and the
resistance on the same side of the fulrum but
the effort is farther from the fulcrum than is the
resistance. Both effort and resistance move in
the same direction.
Third-class levers have the effort applied on the
same side of the fulcrum as the resistance but
the effort is applied between the resistance and
the fulcrum, and both effort and resistance
move in the same direction.
First- and second-class levers magnify the amount
of effort exerted and decrease the speed of
effort. First-class and third-class levers magnify
the distance and the speed of the effort exerted
and decrease its magnitude.
The same general formula applies to all three types
of levers:
—L= —R
l E
Mechanical advantage (M.A.) is an expression of
the ratio of the applied force and the resistance.
It may be written:
Blocks (Pulleys)
#1. How to tell the Mechanical Advantage of a block and tackle:
Here’s a good tip. If you count the number of parts
of rope going to and from the movable block you can
figure the mechanical advantage at a glance. This simple
rule will help you to approximate the mechanical
advantage of most tackles you see in the Navy.
Figure 2-8.-Some other tackles.
#2 You can secure the dead end of the fall to the
movable block. The advantage is increased by 1. Notice
that this is done in figure 2-7. That is a good point to
remember. Remember, also, that the strength of your
fall—rope—is a limiting factor in any tackle.