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Improve math skills of your kids - Learn step-by-step arithmetic from Math games

Math: Unknown - Step-by-step math calculation game for iOS.


Math: Unknown is much more than a math game. It is a step-by-step math calculation game which will teach users how to calculate in the correct order rather than just asking only the final calculated results.

The app consists of four basic arithmetic operations which are addition, subtraction, multiplication and division. In order to get started, users who are new to arithmetic can learn from animated calculation guides showing step-by-step procedures of solving each type of operation. It is also helpful for experienced users as a quick reference.

Generally, addition and subtraction may be difficult for users who just start learning math especially when questions require carrying or borrowing (also called regrouping). The app helps users to visualize the process of carrying and borrowing in the way it will be done on paper. Once users understand how these operations work, they are ready to learn multiplication and division.

For most students, division is considered as the most difficult arithmetic operation to solve. It is a common area of struggle since it requires prior knowledge of both multiplication and subtraction. To help users understand division, the app uses long division to teach all calculation procedures. Relevant multiplication table will be shown beside the question. Users will have to pick a number from the table which go into the dividend. Multiplication of selected number and divisor is automatically calculated, but the users have to do subtraction and drop down the next digit themselves. Learning whole calculation processes will make them master it in no time.

Math: Unknown is a helpful app for students who seriously want to improve arithmetic calculation skills.

Engineering Plastic PEEK Polyetheretherketone

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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…

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 requi…

Engineering Plastic PA Polyamide

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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 …

Engineering Plastic POM Polyacetal

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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,…

Bolts, Nuts, Screws, Studs

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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 no…

Perpetual motion

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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 energ…

Ball Detent Torque Limiter: Overload Clutch

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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 s…

Keyless Bushings for power transmission

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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 MethodsInterference 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…

Column Design (Part 6)

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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 …