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MANUFACTURING TECHNOLOGY


Important differences between the new DIN 25201 and old DIN 65151 New requirements – DIN 25201


Specifies a reference test using an unsecured bolt and verification tests using secured bolts.


Old requirements – DIN 65151 Did not specify test types.


Specifies test fittings, testing frequency and stressing and pre-stressing force. Did not specify test parameters.


Every fastener size must be tested. The results from a Junker Test on one size of bolt cannot be applied to another size.


Requires a washer compliant with DIN EN ISO 7093-1.


The washer surface must be ground to comply with DIN EN ISO 7093-1. Also the shape and flatness must comply with DIN EN ISO 4759-3.


with the roughness also complying with DIN 7093-1. Its shape and flatness must comply with DIN EN ISO 4759-3. The washer’s contact surfaces should be coated with molybdenum disulfide paste and its hardness must be in the 200HV (Vickers Pyramid Number) ranges for strength grades up to 8.8 and 300HV for strength grades above 8.8. During testing, the washer should be fixed in place and not allowed to turn. All these requirements are new and much more demanding than previously. To start the reference test, the fastener


must be placed securely in a Junker Test Bench and the displacement varied until the point when it self-loosens after 300 load cycles, ±100 load cycles at a frequency of 12.5Hz. Once this initial reference test displacement has been discovered, the fastener should undergo three subsequent reference tests to ensure that the effective displacement is correct and the results are consistent. This means running the same test again, but with fresh fastenings and washers and under exactly the same conditions. Maintaining consistency is very


important, because the test report will require the exact test conditions to be precisely recorded. Clearly, just with that covered so far, the new DIN 25201 already exceeds the requirements of the old DIN 65151, but we are just getting started.


Conducting the verification tests The objective of the next stage is to


test the performance of the securing element of the fastener and to find out at what point it starts to loosen. There are some important points to note here. First, there must be 12 verification tests for


Did not restrict test results from one size of bolt being applied to another.


No washer was required. No washer was required.


each set of measurements recorded from the reference test. This is a significantly greater requirement compared to the old DIN 65151. It can also be a painfully slow process with some Junker Test Bench models that require manual operation and set-up. Each verification test must be performed on the fastener, with its locking mechanism in place, either until it loosens or 2,000 load cycles have been completed. The 12 tests must measure and record the following variables:


• The pre-stressing force. • The transverse displacement under load. • The number of load cycles.


Crucially, the test results can only


be used for the specific diameter of the fastener being tested. This might seem like an obvious point, but under the old DIN 65151 testing regime, which did not impose this requirement, many manufacturers only tested the securing effect of their locking device on one diameter of their fastener range. It was then assumed that the results of the Junker Test on a 5/16 bolt, for example, were probably okay to apply to a 7/16 bolt. This is no longer acceptable, meaning fasteners rated on this basis will fail the new DIN. It also means that many manufacturers’ product ranges will require re-testing to the new standard, if they are to remain on specification lists.


Assessing the securing effect Having completed the 12 verification


tests, the next stage is to plot the results of the pre-stressing force against the number of load cycles. The securing effect is then assessed according to whether the


126 Fastener + Fixing Magazine • Issue 71 September 2011


bolt failed before 2,000 load cycles were completed or on the percentage loss of the pre-stressing force after 2,000 load cycles. The DIN says that the securing effect of


the locking mechanism is considered to be ‘adequate’ if there is 80% or more of the pre-stressing force, from when the test started, remaining after 2,000 load cycles. So when demonstrating the performance of locking mechanisms to potential clients, it is important to show that the fastener is still tight in its test bed. Also, based on the plot of the pre-stressing force against the number of load cycles, the pre-stressing force curve must show that it is unlikely that the fastener would have failed if the test continued beyond 2,000 cycles.


The test report demands considerable detail Another major departure from the


requirements of the old DIN 65151 is the reporting procedures. Effectively, what the new DIN 25201 demands is much greater rigor when recording test results. However, despite the extra effort, the benefits of this new approach are considerable. The new test report requirements


will not only provide customers with data about the products they are buying, but will also allow researchers and test engineers to exchange data about locking device performance with much greater confidence. From the outset, it should be known


that detail is a key factor. The test report must first describe which test fittings were utilised in detail and include the number used, their dimensions, material and surface designations and quality


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