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: http://en.wikipedia.org/wiki/Polytetrafluoroethylene

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:

Sunday, October 7, 2012

Perpetual motion

Interesting articles from Wikipedia and other web sites...

From Wikipedia.org
Perpetual motion describes "motion that continues indefinitely without any external source of energy; impossible in practice because of friction." It can also be described as "the motion of a hypothetical machine which, once activated, would run forever unless subject to an external force or to wear". There is a scientific consensus that perpetual motion in an isolated system would violate the first and/or second law of thermodynamics.

Despite the fact that successful isolated system perpetual motion devices are physically impossible in terms of the current understanding of the laws of physics, the pursuit of perpetual motion remains popular.

There is a scientific consensus that perpetual motion in an isolated system violates either the first law of thermodynamics, the second law of thermodynamics, or both. The first law of thermodynamics is essentially a statement of conservation of energy. The second law can be phrased in several different ways, the most intuitive of which is that heat flows spontaneously from hotter to colder places; the most well known statement is that entropy tends to increase, or at the least stay the same; another statement is that no heat engine (an engine which produces work while moving heat from a high temperature to a low temperature) can be more efficient than a Carnot heat engine.

In other words:
  1. In any isolated system, one cannot create new energy (first law of thermodynamics)
  2. The output power of heat engines is always smaller than the input heating power. The rest of the energy is removed as heat at ambient temperature. The efficiency (this is the produced power divided by the input heating power) has a maximum, given by the Carnot efficiency. It is always lower than one
  3. The efficiency of real heat engines is even lower than the Carnot efficiency due to irreversible processes.
The statements 2 and 3 only apply to heat engines. Other types of engines, which convert e.g. mechanical into electromagnetic energy, can, in principle, operate with 100% efficiency.

From Youtube: Some interesting experiments about perpetual motion.

Please, be advised:
These Videos are of motorized versions that were built to illustrate how these machines were supposed to work in the minds of Inventors.

Other interesting resources:

Sunday, May 13, 2012

Ball Detent Torque Limiter: Overload Clutch

A torque limiter is an automatic overload clutch that provides machine protection and reduces repair time during jamming load conditions. This is done to protect expensive machines and prevent physical injuries. A torque limiter may limit the torque by slipping (as in a friction plate slip-clutch), or uncouple the load entirely (as in a shear pin).

Ideally the torque limiter should be placed as close as possible to the source of the jam. This will allow the system inertia and torque to be quickly and effectively disconnected from the jammed section. The system can then be allowed to stop without causing further machine damage. A mechanical torque limiter will provide faster response times and better protection than typical electronic methods at high crash rates.
There are several disconnect types available, but we will focus at the Ball Detent type.

A ball detent type torque limiter transmits force through hardened balls which rest in detents on the shaft and are held in place with springs. An over-torque condition pushes the balls out of their detents, thereby decoupling the shaft. It can have single or multiple detent positions, or a snap acting spring which requires a manual reset. There may be a compression adjustment to adjust the torque limit. Unlike friction style or shear pin type torque limiters, ball detent torque limiter can provide an accurate method of resetting the torque with no operator intervention. A single position clutch will reengage in the exact rotational position each time. This is often necessary for system timing in bottling, packaging, and paper converting type applications.

This is how it works.

  1. When the set limit torque is reached, the clutch disengages; the torque drops immediately
  2. After the cause of overload has been removed, the clutch re-engages automatically after 360 angular degrees. Other cycle sequences, for example 180 degrees, are also available.
  3. The clutch is ready for operation again
The following video clip is from mayr showing how it works to transmit and limit torque. Examples of torque limiter application are provided in the link below with calculation example as a guideline for selecting the right torque limiter model for your mechanical design project.


Sunday, April 29, 2012

Keyless Bushings for power transmission

There are several methods to connect shaft and hub together for power transmission. Let's find the advantage of using keyless bushings from Fenner compared with other traditional connection methods.

Traditional Connection Methods

Interference Fits (Shrink and Press)
A shrink fit is a procedure whereby heat is used to facilitate a mechanical interference fit between two pieces of metal, such as a steel shaft and hub. Extreme heat is applied to the hub, causing it to expand and increasing the size of its machined bore. The expanded hub is removed from the heat source and quickly positioned onto the shaft. As the hub cools, its bore contracts back to its original machined dimension, effectively “shrinking” the hub onto the shaft.

A press fit achieves the same end as a shrink fit — a mechanical interference fit between a steel shaft and hub — but does so through different means. Press fits rely on the application of simple brute force to “press” the hub onto the shaft. Interference fits offer several advantages, such as zero backlash and uniform fit pressures, but these advantages come at a price. High capacity interference fits require long fit lengths, close tolerances, expensive and sometimes hazardous heat sources or hydraulic presses, and field maintenance is extremely difficult. Finally, separated components can rarely be re-used.

Keys, Keyways and Splines
The centuries-old industry standard shaft-to-hub mounting technique is the key and keyway. While ubiquitous and intuitively easy to understand, the key and keyway is a remarkably ineffective technology. Machining a keyway into a shaft is not inexpensive, nor is the equipment required to do so, though these costs are often unknown or overlooked. Keyways introduce notch factors, which account for the reduced effective cross section and abridged fatigue life that occurs when a shaft is keyed and lead, in turn, to systematic over-sizing of shaft diameters. This translates to more shaft material and weight, larger bearings and other drive components, and increased cost.
The combined effect of these clearances is backlash. In applications with frequent starts/stops, direction changes, and/or shock overloads, this backlash can lead to pounded out keyways, fatigue failures, fretting corrosion or some combination of these failure modes. Nor do keys and keyways lend themselves to motion control applications, since backlash erodes the accuracy of motion profiles over time.

A splined connection is simply a series of keys and keyways that suffers the same limitations and drawbacks associated with a single keyed connection. Manufacturing costs are high, especially on hollow shafts, and special surface treatment is often required to increase strength.

Why Go Keyless

Today’s global marketplace demands precise, efficient machines that optimize productivity while minimizing material and fabrication costs. When compared to traditional connection methods, Fenner Drives Keyless Bushings offer the following advantages:
  • A mechanical interference fit with a uniform pressure distribution similar to that achieved through a shrink or press fit.
  • A true zero backlash shaft-to-hub connection with none of the operational drawbacks of keyways or splines.
  • The ability to mount on plain shafting, which need not be
over-sized to compensate for notch factors. This allows the use
 of smaller shafts and bearings for more cost effective designs.
  • The flexibility to mount over existing keyways if desired.
  • Straight bore machining of the mounted component, generous machining tolerances and as-turned surface finishes.
  • Complete axial and radial adjustability.
  • Simple installation, adjustment and removal, even in the field.

Principles of Operation
Though offered in many shapes and sizes, Fenner Drives Keyless Bushings and Specialty Locking Devices all operate using the simple wedge principle. An axial force is applied — by either a hex nut or a series of annular screws — to engage circular steel rings with mating tapers. In the case of keyless bushings, the resulting wedge action creates a radial force on the tapered rings, one of which contracts to squeeze the shaft while the other expands and presses into the component bore.
In the case of specialty locking devices, similar tapered geometry generates a radial force that is concentrated (in the case of Shrink Discs) around a solid steel hub, squeezing so tightly that the hub “shrinks” onto the underlying shaft, or (in the case of WK Series Couplings) simultaneously onto two solid shaft ends to form a high- capacity rigid coupling.
In all cases, the product of the radial force applied to the shaft, the radius of that shaft and the coefficient of friction between the surfaces being joined equals the rated torque capacity of the connection.
Source: http://www.fennerdrives.com/

Saturday, March 24, 2012

Column Design (Part 6)

From Column Design Part 1 to 5, we talked about the formulas to calculate the critical buckling load. This time we're going to use the excel spreadsheet program to help calculate. Let's use the following design problem as an example.

The machine designer would like to calculate the allowable load for his steel column having rectangular cross section. The column has section 80 mm x 30 mm, and 380 mm long. It's proposed to use AISI 1040 hot-rolled steel. The upper end is pinned and the lower end of the column is inserted into a close-fitting socket and is welded securely as can in the picture.

To calculate the critical load for the column we need to do as following.

Solid rectangular section, 80 mm x 30 mm.
The area moment of inertia, I = 1/12 x 80 x 303 = 180000 mm4  -- The least I (for this case, around X-X axis) is used.
The cross sectional area, A = 80 x 30 = 2400 mm2
Then the radius of gyration, r = (180000/2400)1/2 = 8.66 mm
Since the column is pinned at the upper end and welded at the lower end, then the end fixity is fixed-pinned.
The practical value of constant K = 0.8 is used for this kind of end fixity.
The effective of the column, Le = KL = 0.8 x 380 = 304 mm
The slenderness ratio = Le/rmin = 304/8.66 = 35.1

The material AISI 1040 hot-rolled steel has yield strength (Sy) and modulus of elasticity (E) as follows.
Sy = 290 MPa
E = 207 GPa

Then we can calculate the column constant, Cc = (2p2 x 207x109 / 290x106)1/2 = 118.7

Because the slenderness ratio (35.1) is less than the column constant (118.7), the column is short. The J.B. Johnson formula should be used.

The critical buckling load becomes,\
Pcr = (2400x10-6) x 290x106[1-(290x106x35.12)/(4p2  x 207x109)] = 665571 N

With the safety factor N = 3, the allowable load for this column is
Pallow = Pcr/N = 665571/3 = 221.8 kN

As usual, mechanical design handbook provides free excel program that can help you solve your column design quickly. Please download the file for FREE at the link below.

Please make sure that macro is enabled so that you can run the program. Once the program is launched, (1) select the end fixity (2) enter values of cross section, column material and safety factor (3) Click calculate to see the result.

Download FREE Excel File of Column Design.
password: mechanical-design-handbook.blogspot.com