Monday, May 25, 2015

Free Solid Edge 2D Drafting ST7

Siemens provides a free copy of Solid Edge 2D Drafting ST7.
Mechanism design always starts with 2D sketches of linkages. This free software allows users to create sketches with constraints. For example: tangent constraint keeps the cam follower (roller) always touching the cam curve. We can then see how other related linkages move. Variables in the sketch can be linked to Excel. So we can automatically change values of desired variables via VBA code. It also has goal seek function that will change the desired dimension which will give the target value of other dimension. It's very interesting to explore its capability. It will be a great tool for machine designer. We will explore more in later posts.

Download free copy of Solid Edge 2D Drafting ST7

Tuesday, November 20, 2012

Engineering Plastic PEEK Polyetheretherketone

PEEK is an abbreviation for polyetheretherketone, a high performance engineering thermoplastic. It is another engineering plastic which is widely used in machine components since it has very good mechanical strength and dimensional stability, excellent thermal and chemical resistance properties and outstanding resistance to abrasion and dynamic fatigue. Its yield strength is about 92 MPa. Its density is about 1.32 g/cm3 which is 1/6 of steel.

It is a strong and stiff thermoplastic material that is often used in applications where performance at high temperatures is required. PEEK has outstanding chemical resistance as well as resistance to hot water and steam. PEEK is insoluble in all common solvents and is extremely resistant to attack by a very wide range of organic and inorganic chemicals. PEEK can be used continuously to 250°C (for short term, it can operate at 300°C) and in hot water or steam without permanent loss in physical properties. It also has high abrasion and cut through resistance combined with low coefficient of friction. So it is used a lot in sliding applications such as bearing, guide, etc. Unlike Polyamide or Nylon, PEEK has excellent hydrolysis resistance. It has very low moisture absorption.

Applications: Gears, wear strips, bushes, metering pumps, pump housings, light mountings, friction bearings, ball valve seals, wafer supports, plug parts.

PEEK is naturally tan in color and can be pigmented with a wide range of colors, allowing for easy part identification.

  • Excellent chemical resistance
  • Very low moisture absorption
  • Inherently good wear and abrasion resistance
  • Unaffected by continuous exposure to hot water or steam. Very good hydrolysis resistant, even against super heated steam
  • Very good dimensional stability
  • High thermal mechanical bearing strength
  • Creep resistant
  • Low smoke and toxic gas emissions.
  • Excellent resistance against high energy radiation. Absorbing more than 1000 M rads of irradiation with no significant reduction in mechanical properties

Monday, November 19, 2012

Engineering Plastic PTFE Teflon

Polytetrafluoroethylene (PTFE) is another engineering plastic using in machine parts. PTFE is a non-stick substance. The trade name of the DuPont company for PTFE is Teflon. PTFE is used as a non-stick coating for pans and other cookware. It has exceptional resistance to high temperatures, chemical reaction, corrosion, and stress-cracking. Working temperature of PTFE is between -200oC and 260oC. Since it has very low friction (PTFE's coefficient of friction is 0.05 to 0.10) and good resistance to chemical reaction, it's normally used in bearings, pipes or containers of chemical substances. It's also used for sealing purpose at high temperature environment. The tapes for thread sealing in pipes are also made from PTFE. Though it can work well until 260oC, but its strength is not always high since it may expand and deform at high temperature. PTFE is easy to machine, low cost and long lasting. PTFE parts are often used in mechanisms that rotate or slide. It doesn't require surface finish and continue to look good and work well.

Example of PTFE parts: Bearings, Sleeves, Bushings, Gears, Cams, Tubing, Electrical insulators, Electrical connectors, Seals, Slide plates, etc.

Further reading:

Sunday, November 18, 2012

Engineering Plastic PA Polyamide

PA is the abbreviation of Polyamide. The commercial name of polyamide is Nylon. Nylon is an engineering thermoplastic which is commonly used in textiles, automotive, carpet and sportswear due to its extreme durability and strength. It has tensile strength about 80 MPa which is higher than POM. Solid nylon is used for mechanical parts such as machine screws, gears, cams, bearings, guides, rollers, and other low- to medium-stress components previously cast in metal. Nylon is frequently used as a replacement for bronze, brass, aluminium, steel and other metals, as well as other plastics, wood, and rubber.

Engineering-grade nylon is processed by extrusion, casting, and injection molding. Nylon price is approximately 2 times of the price of POM (Polyacetal). Working temperature when using Nylon should be within - 50oC to 160oC. Nylons are available in various colors such as white, blue, black, etc. One important property of Nylon that should be noted when use it to make machine components is moisture absorption. Moisture has significant effect on properties of Nylon. It may change in size which may cause problems.

Notable material features:
  • Very good physical properties. High tensile strength and modulus of elasticity
  • High impact resistance, a high heat distortion temperature. Very good heat resistance.
  • Excellent wear resistance. Resists wear, abrasion, and vibration
  • Excellent chemical resistance. Can withstand contact with chemicals, alkalies, dilute acids or oxidizing agents.
  • Nylon is NOT moisture Resistant. Moisture has significant effect on properties.
  • Moderate to high price
  • Fair to easy processing
  • Electrical connectors
  • Gear, slide, cams and bearings
  • Cable ties and film packaging
  • Fluid reservoirs
  • Automotive oil pans

Tuesday, November 13, 2012

Engineering Plastic POM Polyacetal

Normally, we use steels to make machine components. But in some applications, engineering plastics are also commonly used in machine parts since their unique properties e.g. lighter weight, easy for machining, great quantities can be made quickly by injection, etc.

General properties of engineering plastics:
Light weight - The density of engineering plastics is usually about 1-2 g/cm3. We often use them in the portable machines that we often move them, or use in the parts that the user has to hold by hand.

No rust - We normally use engineering plastics without surface treatment to prevent rust. However, engineering plastics do not withstand the corrosion from oxygen or ultraviolet, so we need to consider this point when using plastic for long time.

Low electrical conductivity - We use plastics to prevent users from direct contact with electrical parts for safety reason.

Low thermal conductivity - Engineering plastics are good for insulation. We can prevent some heat from machine. However, they usually cannot withstand high temperature and they may deform and strength reduced.

POM or Polyoxymethylene , also known as acetal, polyacetal and polyformaldehyde is an engineering thermoplastic used in precision parts that require high stiffness, low friction and excellent dimensional stability. We use POM to make gears, cams, bearings, guides, rollers, etc. It normally has milky color or black color. Its tensile strength is about 60 MPa which is 1/7 of the strength of SS400 steel and the weight is about 1/7 of steel. It can be used for sliding application. Working temperature should be within - 40oC to 105oC.

Saturday, November 10, 2012

Bolts, Nuts, Screws, Studs

Screws and bolts are the most common types. The difference between screws and bolts is only the intended use. Screws are intended by screwing into tapped holes; bolts are intended for use with nuts. Sometimes screws are supplied with a captive washer under the screw head. They are also called SEMS. A SEMS screw is a generic term used to describe a screw pre-assembled with a free-spinning lock washer. It is a permanent assembly with the washer held in place by the major diameter of the screw thread being larger than the hole of the washer. The SEMS screw is simple, easy to use and available in many different styles making it well suited for many applications. SEMS save assembly time and eliminate the possibility that a screw will be installed without its specified washer. Normally, a bolt can also serve as a screw by using it with a tapped hole rather than a nut.
A stud is threaded on both ends. It is usually screwed permanently into a tapped hole. Threads on the two ends may or may not be identical. A threaded rod is the least common type. It is usually used when a very long threaded member is needed. It can often be purchased in a long length and then cut off as required.

An advantage of threaded fasteners in comparison with riveting, welding, etc., is that they can be easily and nondestructively disassembled. However, a disadvantage of threaded fasteners is that they sometimes loosen and disassemble themselves. The relative motion between the bolt and nut threads  is often caused by vibration, but it can have other cause such as differential thermal expansion of both parts.

The factors influencing whether or not threads loosen are:
  • The greater helix angle, the greater the loosening tendency. Thus, coarse threads tend to loosen more easily than fine threads.
  • The greater the initial tightening, the greater the frictional force that must be overcome to initiate loosening.
  • Soft or rough clamping surfaces tend to promote slight plastic flow which decreases the initial tightening tension and thus promotes loosening.
  • Surface treatments and conditions that tend to increase the friction coefficient provide increased resistance to loosening.
Source: Fundamentals of machine component design, Robert C. Juvinall & Kurt M. Marshek

Recommended fasteners related resources: