Part One of this series can be found here.

In our previous article we discussed capscrews, bolts, and nuts along with their relative hardness and thread sizes. In this segment we will finish up on our fasteners and then work with ways to keep them from loosening up in the field.

Capscrews, bolts and nuts are not the only means of holding two parts together. When dealing with thinner metals like sheet tin, a long bolt and nut would be a little redundant since the actual parts fastened are so thin. This is where sheet metal screws come in handy. They are quick and inexpensive.

With the import of cheap overseas hardware it would seem that sheet metal screws come in an ever-changing assortment of head types. In addition to the standard phillips and flat head version one can find fluted, clutched, allen, torx and who knows what else. I suppose they all work as well as the tool that installs them, but for our purposes the standard flat and phillips installed with screwdrivers is what we’ll work with.

Put simply sheet metal screws are for attaching things to sheet metal. The screws can be inserted into predrilled holes and, as the screw is tightened down, its deep threads grip the sides of the hole and pull the item tight. If no hole exists then one can be drilled. Use a drill size equivalent to the diameter of the unthreaded shank of the screw. Don’t wallow out the hole with the drill as this will reduce the surface area for the screw to grip by making the hole oblong. A wonderful invention when using screws in un-prepunched metal are the self-drilling type. These screws have a small drill bit molded onto the tip. When inserted into the driver of a power screwdriver they drill the proper hole and tighten the screw in one operation. They are fast, slick, tidy, and easy to use. If one does not have access to a power drill then a sharp punch and hammer work quite well for creating the holes. Punching a hole in the sheet metal creates a kind of crater in the surface for the screw to penetrate. When inserted and tightened down the screw grips the crater’s sides and actually tries to pull the sides back together. Believe it or not, this action creates a tighter fit than a drilled hole and will hold better as there is more gripping surface between the screw threads and the receiver metal.

When using metal that is a little bit thicker than sheet tin one can predrill the hole and then install a self-tapping sheet metal screw. These screws have cutting threads on the tip of the screw and act like a small tap when inserting into the hole. They hold tightly and work well.

Let’s back up for a moment and take another look at nuts and bolts. A tractor in a field can produce vibrations, stresses, and heavy loads that make nuts and capscrews loosen. Lets examine how to keep that from happening.

Manufacturers have developed a number of devices to keep nuts in place. Special washers, locking nuts, plastic collars, and liquid fasteners are a few of the types we have to work with. Probably the most common thing used on the farm would be locking washers. They are designed to provide a friction point between the nut and part surface. When using these washers on softer metal like aluminum place a plain flat steel washer underneath to prevent damaging the part. Make sure the flat washer chamfer is up and the flat side is down towards the part. Two other types of locking washers are the internal and external. They work well on sheet tin and with screws. These types might be used in applications where a lower torque value is required as they may crack and fall apart under high pressure.

There are a few types of locking nuts that are worth mentioning. A palnut is made from thin stamped metal and is designed to bind against the threads of the bolt when installed. Spin it down in contact with the nut (open side away from nut) with the fingers. When firmly in place tighten an additional one half turn to secure. A collar nut looks like a standard nut with a plastic or nylon ring on the inside of the top threads. When in place and tightened it provides friction to the bolt threads via the plastic that does quite well in jamming the nut to the bolt. A similar slotted and pinched nut uses a raised threaded section of nut that is smaller than the rest of the threaded portion. The bolt forces the slotted section apart providing enough pressure to keep it from vibrating loose. Other types of locking nuts include a crimped nut and a distorted thread nut. These have crimped ends that effectively jam the nut to the bolt threads.

There are many other types of restraining devices used for securing nuts that I will only mention in passing. Cotter keys and rolls pins insert through a drilled hole in the bolt or clevis. Use as thick a cotter key as possible and cut off the surplus end. Roll pins should be installed using a round punch and hammer. Safety wire inserts through capscrew heads or drilled nuts. When properly installed and tightened they provide tension that is opposite to the loosening rotation of the fastener. Locking plates have tabs that are bent up onto a nut or bolt face thus holding the fastener in position. Setscrews usually lock the item to a flat spot ground onto the threaded shaft.

Take note of the proper locking device for the application when disassembling a project. Parts manuals are quite handy when reinstalling components according to specs as they, often times, include all fasteners and locking devices in the schematics. As mentioned earlier the manufacturers have gone through great expense to ensure the correct fastener for their component. Make use of their recommendations and requirements to get the proper life and safety out of your piece of equipment.

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