80 TIME COMPRESSION
Statistical software helps evaluates the reliability of products I
n today’s business environment, design engineers constantly need to evaluate the reliability of their products. The ability to define the probability that a product will perform its purpose for a specified period of time under precise environmental conditions is one type of reliability method which could be used. Through a Demonstration Test Plan using
Minitab Statistical Software, it is possible to prove, through testing, that a reliability specification or a redesigned system has been improved for reliability through sample size, test time and number of failures. This method verifies that only a certain number of failures occur in a set amount of time. There are two different types of tests:
n Substantiation tests provide statistical evidence that a redesigned system has
suppressed or significantly reduced a known cause of failure, eg proving that a redesigned jet engine turbo pump is better than the original. n Reliability tests provide statistical evidence that a reliability specification has been achieved, eg if the reliability of the turbine engine combustor at 2000 cycles exceed 99 per cent. Here is an example: To give a working example, a thermostat has been redesigned to increase its reliability. Six thermostats are available for testing. The design engineer will need to determine how much time they will need to test these units, with 0 failures, so that they can be 95 per cent confident that the time at the 5th percentile is at least 421 hours. The failure times of the original thermostat fit a Weibull distribution with a shape of β = 1.9. Using the hypotheses:
n H0: 5th per centile = 421 hours. n H1: 5th per centile > 421 hours. Maximum number of failures
Entering 0 results in the smallest sample size and test time. However, if just one failure occurs, the demonstration test fails and you cannot conclude that you have achieved the desired reliability.
Modelling software speeds design of organis shapes
ensable Technologies is releasing a major new version of its Freeform 3D modelling software. With new design functionality and expanded interoperability, the new Freeform version 10 builds upon its strengths for designing complex organically-shaped products with enhancements that let product designers strike a balance between aesthetics and manufacturability. Cost-effective manufacturing of goods with organic shapes or sculptural design details and textures is exceedingly complex to achieve. The new Freeform version provides designers with tremendous speed and creative freedom to refine their designs while streamlining the preparation-for-manufacturing process. Freeform is currently said to be used by thousands of product designers in a wide range of industries including toys, footwear, home décor and collectibles, custom medical implants and many more. New features allow faster design iteration and preparation of intricate models with precise surface details and textures to meet downstream manufacturing requirements for cost-effective computer- aided manufacturing such as rapid prototyping/manufacturing or milling. For example, designers can quickly calculate the surface area of decorative items that will be gold plated while they sculpt
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– allowing them to keep the cost of materials within specification. Additionally, designers can fix draft angles – required for mould release – to select areas of organic models in seconds, typically a time-consuming or impossible task in parametric modellers. With Freeform, users model more intuitively through the use of haptics (touch-enabling) for product design. Instead of holding a computer mouse, Freeform users hold a Phantom force-feedback haptic device – literally 'feeling' the resistance on their hand as they nimbly sculpt intricate designs, such as grapes and vines or apple blossoms on juice bottles. Because it is based on voxels (volumetric pixels), Freeform also removes the constraints of topology – mathematical definition, geometry format, and order of operation – of traditional mathematical-based modellers, which further enhances its ease of use. New features in Freeform version 10 include a advanced toolsets such as deform, bend, twist, inflate and deflate, and no-fail geometry that is 'water-tight' for use in CAM software programs; 'pinch' and 'crease' digital clay to add intricate forms and details. n
For more information, visit
www.sensable.com www.engineerlive.com
Sample sizes or testing times The choice may be made as to whether to fix the sample size and determine the test time for each unit or the reverse option. Interpreting the results the figure shows
the Minitab Statistical Software results, while also displaying the sample size and testing time above the graph. This shows that six units each need to survive 1395 hours to demonstrate the desired reliability. Ratio of improvement The x-axis in the graph is the ratio of improvement from the specified desired reliability. For example, a ratio of improvement of four means the true 5th per centile is 4*421 or 1684 hours. The graph displays the likelihood of demonstrating the desired reliability for different ratios of improvement. The likelihood of passing the test appears as a percentage, this is the power of the test. In this particular example, the power is greater than 0.80 (a common reference point) for ratios of improvement greater than four. This means that the probability that you will pass the demonstration test when you have a real 5th per centile of more than 4*421 or 1,684 hours is greater than 80 per cent. n
Minitab Ltd is based Binley
www.minitab.co.uk
Enter 80 or ✔ at
www.engineerlive.com/ede , Coventry
, UK.
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