Special insert gives protection in potentially explosive environment

Tappex Thread Inserts have a wide range of standard inserts in stock to meet most customers performance and installation requirements.

In some cases, the application specification can mean that a variant longer, shorter, with a special thread or other feature, needs to be developed to satisfy the requirements. Alternatively, an entirely special part can be developed in conjunction with the customer and the needs of the application.

Insert modifed for task in hand

Tappex was recently approached by a customer with such an application and modified its Hi-Mould moulded in insert to meet their requirements.

An existing moulded in insert had been trialled where a plastic enclosure was required to withstand an internal explosion. The existing insert came close to meeting the requirements, but, during the various test cycles, one insert had failed. Thread-forming screws had been trialled with some success, but industry standards for the application meant that they could not be used. Tappex was asked to develop a solution.

Restricted space

Unfortunately, the space surrounding the insert was restricted, so a simple increase in length was not possible as a means of increasing the resistance to pull-out. After discussions with the customer, two variants of the existing insert were tested with larger diameters and undercut forms without success.

In an attempt to give some direction to the project, a range of inserts of different types was moulded in and then tested. As a means of comparing the inserts, a simple jack-out test was devised where torque was applied to the unsupported insert. The torque at which the insert failed was then recorded.

Results showed that the oversize self-tapping style insert, with no cutting features, offered the greatest performance gain in pull-out.

Combination of features used

To maximise the benefit of the enhanced pull-out performance, while retaining an acceptable level of rotational performance, a combination of features of a self-tapping insert and mould in insert was designed and samples made for testing at the customer.

“The special bespoke design of this thread insert exceeded the customer’s required performance,” says Tappex. “Prior to production, the design was also tweaked to increase the bolt engagement length by 1mm. This ensured that the bolt could be tightened. without any risk of bottoming in the insert.”