Introduction
Well we all know what screws are, but nevertheless we’ll provide you with the bookish definition.
A short, slender, sharp-pointed metal pin with a raised helical thread running around it and a slotted head, used to join things together.
The cylindrical portion of the screw from the underside of the head to the tip is known as the shank; it may be fully threaded or partially threaded. The distance between each thread is called the pitch.
The majority of screws are tightened by clockwise rotation, which is termed a right-hand thread; a common mnemonic device for remembering this when working with screws or bolts is righty-tighty, lefty-loosey.
Screws v/s Bolts
A bolt is an externally threaded fastener designed for insertion through holes in assembled parts, and is normally intended to be tightened or released by torquing a nut.
A screw is an externally threaded fastener capable of being inserted into holes in assembled parts, of mating with a preformed internal thread or forming its own thread, and of being tightened or released by torquing the head.
Selection Criteria for Screw Material
- Size
- Required strength
- Resistance to corrosion
- Joint material
- Cost and temperature.
- Mass
Screw Heads
Screw Drives
A screw drive is a pattern which is used to turn a screw. It allows torque to be applied to it usually by using a mating tool, such as a screwdriver, that is used to turn it.
Phillips drive
Phillips drives are the most common type of screw drive. They were developed as one of the first alternatives to the slotted drive.
Slotted Drive
The slotted drive is the oldest drive still commonly used today. They provide less torque than other drives and are subject to slippage.
Socket Drive
Socket drive screws contain a hex shaped internal socket which is tightened or loosened using a hex key or allen wrench.
Hex Drive
One of the few drives turned by using the outside of the head rather than the inside. Hex head screws are installed with a socket, usually connected to a ratcheting tool or a power driver.
Robertson Drive
Square drives have a square shaped socket which has a slight taper as it goes deeper into the screw head. The four 90° angles provide strong torque and decrease the possibility of slippage.
U Drive
A U drive screw is also known as a hammer drive screw. It is set in a properly sized pilot hole and then driven into place with a hammer. The threads of the screw turn as it is driven, creating the fastening action.
Other types of screw drives are shown in the image below:
Selection criteria of the screw for satellite
Screws are used to attach any two surfaces in contact on our satellite. The size of the screw will be decided by the maximum depth of penetration of the hole. The screws are generally made of the material SS304 (stainless steel).
Single slotted and Phillip screws tend to slip off while using a screwdriver to screw them.These fasteners are easily damaged at the slot. In addition, these are hard to use with a torque wrench.
Allen/hex head fasteners are very common in most equipment. These are more durable and less likely to slip. This also allows use of torque wrenches. Hence, allen headed screws are a good alternatives.
How tight should a screw be?
The tighter the screw, the more friction there is to resist loosening.
Screws are very likely to come loose if any relative motion occurs between the threads. Static friction is substantially higher than sliding friction, so once movement in one direction starts, it becomes much easier for unscrewing motion to happen.
As a follow up, bolts are less resistant to shear (sideways) loads, so more clamping force and friction helps reduce these loads.
As long as joint separation does not occur, additional tightening preload does not buy any additional fatigue protection.
How to achieve the required tightness?
According to the Machinery's handbook, tightening by feel is only +-35% accurate, and using a torque wrench only improves the accuracy to about +-25%. These uncertainties are massive, and give good reason not to tighten too close to yield, or too loose, and also to design a joint so that it will still work with a huge span of possible pre-loads. A method called "turn-of-nut" can supposedly get within +-10%, but this relies heavily on a reliable starting point from which to start counting turns (see Machinery's handbook).
Additional Information
SAE J429 defines the bolt grades for inch-system sized bolts and screws. It defines them by grade, which ranges from 0 to 8, with 8 being the strongest. Higher grades do not exist within the specification. SAE grades 5 and 8 are the most common.
Metric ISO Marking
Metric fasteners are marked with two numbers separated by a decimal point, like 10.9. The 10 is 1/100th of tensile strength in MPa, and the .9 represents the ratio of yield to tensile strength. So 10.9 represents a tensile strength of 1000 MPa and yield of 900 MPa.
References
- http://www.timco.co.uk/screw-types
- http://store.curiousinventor.com/guides/Metal_Working/Screws
- https://en.wikipedia.org/wiki/Screw
- Unbrako Engineering Guide"
- Fundamentals of Machine Component Design," by Robert C. Juvinall, Kurt M. Marchek
- The Machinery's Handbook, 27th ed.
- "Handbook of Bolts and Bolted Joints," by John Herbert Bickford, Sayed Nassar
- "What Every Engineer Should Know About Threaded Fasteners: Materials and Design," by Alexander Blake
- http://esmat.esa.int/ecss-q-st-70-46c.pdf
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