Shear Knowledge

To Keep Business from Walking Away, Distributors Need to Suggest Best-Choice Options for Fasteners

By Thomas Doppke -- Industrial Distribution, 6/1/2006 6:00:00 AM

The first part of this article, published in June 2005, discussed the process by which a fastener is selected. We looked at choosing bolt and nut, versus rivet and screw, for particular applications. We discussed the differences between various bolt/nut, bolt and integral washer, nut/washer, and nut types and where each is useful. In this installment, we will look at screws, washers and some of the other hardware encountered in fastener engineering design.

Distributors are being asked, more and more, not only to supply fasteners, but to actually engineer the part that is required. Customers no longer simply send in orders; they ask fastener distributors for application advice. The customer can cut costs by having his suppliers do some of this engineering work—and to retain the customer, the distributor must be able to make the best recommendation for the application.

Sounds like a nightmare of decisions? Not so, if the distributor knows the advantages and disadvantages of various fasteners. Just about any part can be designed to assemble, but understanding some of the basics of those parts, and what they can be combined with, can prove invaluable to customers.

The difference between a screw and a bolt is not clearly defined. However, a standard definition (and there are numerous exceptions) is that a screw is capable of making its own mating thread and bolt threads into a pre-threaded hole. The screw hole is usually piloted (pre-drilled to a certain size). Screws were originally designed for attaching into wood and had a tapered body, a sharp point for starting and some type of drive mechanism. Square heads were initially used.

Sheet metal came into use during the expansion of technology about the turn of the century. The World Wars speeded up the mass production of goods, which, in turn, meant mass assembly. The tapered body wood screws did not hold well in sheet metal and a straight body screw form became the standard for almost 100 years.

Screws fall into four general classifications. The first is the standard sheet metal screw, with a straight body, point and "spaced" threads. That is, the threads are spaced apart to allow for metal between the adjacent threads, which improves pull-out resistance. The second is a bolt-like part (fine thread) with a cutting feature added to tap a thread. Third is the thread-rolling screw, which does not cut a thread but rolls it instead. Finally, there are miscellaneous special threads and parts used for unique applications.

Standard sheet metal screws are inexpensive, readily available and are well-known hardware. They have moderate tensile strength and resist pull-out well. Vibrational loosening and retention strength are problems when used in thin metal.

The screw develops full strength when tapped into metal at least one thread pitch in thickness. The screw type is also limited in the other direction. As the thickness increases, the hole diameter must increase to prevent frictional resistance from seizing the screw and either jamming it or breaking the part if the torsional load is too great. However, there are limits to how large the hole can get and still have any gripping power.

If the metal is thick, the use of a pre-drilled hole and a bolt-thread fastener (fine-thread as opposed to spaced-thread) is often used. This type cuts its own thread into thick metal and produces a joint as good as a regular bolt/nut joint.

Fine-thread tapping screws are also used as paint cleaners and to remove dross from threaded holes. However, they can become clogged with metal or paint, often within one turn, so the part no longer "cleans."

The thread-rolling screw was invented for two main reasons. The part does not produce cuttings that could fall into the assembly (electronics, moving motors, etc.) and secondly, the part forms a thread that has a prevailing torque resistance. That is, the part taps its own hole and has a locking feature as well.

Plastics and threads

Special materials require special fasteners. Plastic can be best described as a slow moving liquid. It will cold flow away from any tightened joint over a period of time. Loosening is the result of this cold flow. Certain plastics have low cold flow rates.

Unfortunately, the most used ones are not in this category. Nylon, polyethylene and propylene, the rubbery ones, are rapid cold flow types. The fastener that must be attached to and into such plastics requires a special thread. Often, a standard sheet metal screw is used for the sake of cost or convenience but it may not perform as desired. In harder plastics, it may crack the attachment site either immediately or after a period of time if the site is highly overstressed. Standard sheet metal screws are used because they are inexpensive, and failures in the locations where they are placed (toys and inexpensive hardware) do not generate a great number of returns and complaints.

The most commonly encountered fasteners for plastic are the dual thread, high and low thread tapping screw, and a push-in type that has a tapered leading thread side for ease of insertion and a backside of the threads at an acute angle for maximum retention. This type can be installed by pushing it into a hole but can be removed, if necessary, with a screwdriver.

This screw comes in cutting and non-cutting versions. The softer plastics are pushed aside by the screw threads and reflow back to give a good locking action. Harder plastics and those with reinforcement (glass fibers, talcs, etc.) require the screw threads to cut a thread into the material. The threads that are formed are somewhat weaker.

Where a screw is needed part of the time, as in an "add-on" option, the addition of a hole to every assembly can be costly and the effort to plug it when it is not needed is also cost prohibitive. The invention of the self-drilling screw solved that problem. It drills and taps its own hole. The parts are more expensive, and many users complain about the issue of safety to operators whose hands must be in the location of the sharp drill point protrusion. Also, the screw tends to skip and damage painted surfaces if not installed correctly.

Washers and rivets

Washers are among the oldest fasteners in the world. Used in prehistoric times for load spreading, bearing surface increase, to prevent rubbing of parts, and to span slots and large holes, they perform much the same functions today.

Standard washers come in several styles and heat treatments. Flat parts are soft for cost reasons and are used where just a simple load or span is needed in joints that are not critical. Hardened, high-carbon steel parts offer increased strength and wear resistance.

Split lock washers (sometimes called helical spring lock washers) offer very little in a modern joint. They are a carryover from the days of wooden vehicles. They offer a slight spring effect but only if the joint is very low loaded, the bolt is long in length, and low-torqued. They can be used in plastics and wood but offer no use in a steel joint.

The conical washer is a spring-loaded part but is almost never found loose. It is a part of bolt and washer/nut and washer assemblies where the washer adds to the joint tension to retard loosening at loads up to one-third of the total load. Above that point, they will be flattened and act as a regular flat washer.

Rivets are inexpensive. Some types are easy to install, and all are fairly simple to understand. The types available are: solid, semi-tubular/tubular, pierce, blind break mandrel, and multi-part break rivets. Solid rivets are pins that are inserted into or through a hole and the inserted end is flattened or otherwise deformed to prevent the rivet from loosening or coming out. All rivets are shear fasteners. That is, they sustain the load well if it is in the shear direction (across the fastener) as opposed to bolts, which are loaded along the direction of the part (tensile loaded).

Solid rivets are installed by a squeezing or hammering blow to the inserted end, forming a mushroomed tail. The components being held together should be tightly clamped to prevent any movement. Loosely clamped parts will not be secure and will fail with fatigue, vibration, wear, overload, and shearing of the rivet. The tooling used depends upon the size of the rivet and its material.

Common errors made when using rivets are:

1. Hole size is too large. The rivet expands inside the clearance hole until it fills the space entirely. This is what prevents movement of the part. Too large a hole may cause the rivet to buckle internally rather than expanding.

2. Plate not clamped tightly. The rivet may expand between the plates, forcing them apart.

3. Rivet too short for application. There will be insufficient material left for the tail cap to form correctly. This may cause the rivet to pull loose.

4. Positional errors. Too close to the edge, too close to each other, or too far apart.

The proper hole diameter is 107 percent of the rivet diameter and the length is application determined. A good rule of thumb is that the length should be 1.3 to 1.7 times the rivet diameter plus the thickness of the materials being joined.

Tubular rivets are small and are usually used as a pivot. They are lightweight and effective and easily installed with a simple arbor press. The difference between the semi and full tubular rivet is the depth of the internal cavity. The semi type, having more solid body, has a higher shear strength.

Pierce rivets require specialized tooling to install and the parts are usually purchased from the tool manufacturer. Little demand is seen for these parts. The rivet is driven into the material being joined. The rivet "splits" apart inside the panels being joined, forming a bifurcated attachment which does not perforate the lower panel.

Blind Break Mandrel Rivets are known by the term "POP" rivets. "Pop" is the trade name for this type of rivet made by Pop Rivet Division, Emhart Product Corp, Division of Black & Decker. There are more than 50 companies worldwide making this type of part. They range from 1/16 in. diameter to ½ in.

Many multi-part rivets are used for very high strength joining (truck frames, military aircraft). The rivet nail is passed through the panels to be joined. A collar is placed on the nail and the tool pulls the collar up to the specified tightness. The tool then swages the collar onto the nail, forming a permanent joint.

Chart 1

Tapping Screw Properties

	Spaced Thread	Cutting Thread	Forming Thread
Taps into thin metal (,0.055 in./1.4mm)	Fair	Poor	Fair
Taps thick metal (0.120–0.320 in./3–8mm)	Poor	Fair	Good
Drive to strip ration better than 1:3	Yes	Yes/No	Yes
Drive effort	Good	Fair	Fair
Strip easily	Yes	Yes	No
Reuse	Fair-Poor	Good	Good
Cost of part	Low	Medium	Med/High
Cost of installation-all factors	Low	Low	Medium
Availability-sizes, types, styles	Good	Fair	Fair

Chart 2

Properties of Screws for Plastics

	Dual Lead High & Low	Lobular	Radius Root	Special Form	Standard Tapping
Use in all plastics except foams	Good	Fair	Fair/Good	N/A	Fair/Poor
Use in foams	Poor	Poor	Poor	Good	Poor
Unique hole size needed	Yes	Yes	Yes	Yes	No
Reuse	Fair	Good	Fair	Fair	Poor
Cost of part	Low	Medium	Medium	High	Low
Vibration resistant	Fair	Good	Good	N/A	Poor
Torsional strength	Poor	Good	Good	Depend on part	Fair/Poor

Chart 3

Drilling Screw vs. Tapping Screw Properties

	Drilling Screw	Tapping Screw
Need hole always	No	Yes
Hole needs to be perfect (size, clean, deburred)	No	Yes
Part needs to be aligned accurately	No	Yes
Tooling sensitive	Yes	No
Limitation of installation thickness	Some	Some
Installation Cost-Labor (all factors)	Low	Medium
Part cost	High	Low
Needs increased mass, backside space	Yes	No
Increased ergonomic problems	Yes	No