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The mighty, mysterious, and magical screw.

The screw is the only mechanical component that even comes close to the wheel or lever both in frequency of use or value. A screw is a simple machine consisting of a cylinder or cone with a narrow inclined plane or "thread" wound in spiral form along its length. Unless it is a common screw with a tapered shape it is fitted into a nut or receptacle of some kind that has been made with a counter thread or hollow spiral to receive the screw.

A screw jack bolt and nutor vice is a perfect example of the screws amazing ability. When force is applied to the end of a lever that is attached to the screw, its continuous inclined plane passes through a stationary nut, pushing before it the weight to be lifted or the resistance to be over come. To illustrate the power potential I will use this example. A screw with 12 threads per inch and 30 pounds applied to the end of a handle that extends14" from the center of the screw will translate into and amazing 31,680 pounds of pressure! Friction often accounts for a loss of force, in this case about ¾ so the actual force applied is about 7920 pounds. That's a big loss but still pretty impressive!

A screw is also pretty handy at holding things together. It would be kind of difficult to assemble an appliance with nails (which hold by friction) it would be even more difficult to take it apart and put it back together again. The mechanical principals behind screws are both fascinating and complicated so ill try not to bore you and get to the point. Appliance manufacturers use a few main types of screws and bolts, knowing the difference between them will make it a lot easier to figure out what screw went were when you are trying to put your appliance back together again. It's also good idea to snap a few pictures before your saying to yourself…"Gee I wish I had snapped a few pictures."

Heads and Threads

Here a few examples of common screw head and thread types used to hold your appliances together while they work their magic.

screw heads

A few things to note about screw heads...

  1. 1. Phillips and Square screws got together and made baby screws called combos. As the name implies you can use ether driver to operate this crossover screw type, however it's usually best to use a square driver because phillips will often slip and cause the screw head to become warn or striped making it increasingly difficult to use. In extreme cases the screw may need to be drilled out to be removed.
  2. 2. Allen and Torx can look very similar in real life at a glance or in dark places. Using an Allan wrench on a Torx head screw might work but it also may slip and strip the head.
  3. 3. Security Torx are used to attempt to keep people out...thus the use of the word security. They are often used on commercial laundry appliances to keep people from tampering with the mechanics of a washer or dryer in the laundromat. Security Torx are also used on microwave ovens to keep the unexperienced tinkerer out of harms way. (The microwave's hi-voltage capacitor can put you on the floor even if the appliance is unplugged!)

These are the more common thread types that you will encounter when servicing your home appliances.

thread types

 

A few things to note about screw threads...

  1. 1. The first four screw types above are considered course thread. Course threads are very versatile in their use for appliance production, they can be used to secure anything from the sheet metal of an appliance cabinet to attaching plastic parts such as control boards, and lid switches. The extremely course threaded screws, such as the second screw are almost always used for securing plastic parts.
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  2. 2. When the uppermost portion of a screw in not threaded it is designed to pass through the first component to be secured and thread into a lower component. For example a screw similar to the third example above is often used to pass through the door panel of a dishwasher and attach the upper console.
  3. 3. The third and fourth examples are considered to be "self tapping" meaning that they are designed to cut new threads as they are inserted into a part. If this style of screw is being used to cut new threads into a metal part you may be required to pre-drill a hole that is just slightly smaller than the screw being used. You may also need to use the power of a drill to drive the screw.
  4. 4.The last three examples are "fine thread" also called "machine thread" These fasteners are for a specific nut or pre-threaded part. This type of screw or bolt is generally used on machined parts that need to be perfectly positioned to operate properly, or securely tightened, such as washing machine transmissions. It is always a good idea to hand start a fastener of this type a few turns before using a driver to avoid cross threading.
  5. 5. If you are super observant you may have noticed the markings on top of the last two bolts. The three marks indicate a grade 5 bolt while the six marks indicate a grade 8 bolt. More markings indicate a higher grade bolt with a higher strength and holding rating.
  6. All this talk about screws reminded me I needed to take care of something...