Variable collets extract all bushes

Tom Shelley reports on a clever way of extracting different sized interference fit bushes and bearings from bores without damaging them


A variable collet can be made up from a chain of interlocking elements to form partly collapsible rings of any size, which may be used to achieve the damage free removal of a wide range of interference fit bearings and bushes.

Key to the patented 'Bushout' system, developed by Harlow based aircraft maintenance specialists, Hanley Smith, is a modular asymmetric element, a thin sandwich of metal, neoprene and metal. The metal ends are designed to interlock, forming a continuous flexible chain. Interlocking the first and last elements of a chain forms an annulus, the collet, the diameter of which is varied by changing the number of elements in it.

Once a collet has been formed by using such a number of elements that will allow a snug fit inside the bush, it may be collapsed and passed down inside it. It is then opened up to engage the outer end of the bush. The bush is may then be extracted using hydraulic force from a hollow cylinder ram.

In addition to providing adequate peripheral 'give' to accommodate bush diameters requiring less than one full element, the neoprene provides sufficient radial tension for a snug fit, the metal providing the axial strength necessary during bush removal. An overhanging 2mm nib capable of withstanding a 2.75tonne load engages the bush.

The ram withdraws the collet with the bush from its housing towards and into a reaction plate, which is held against the bush/bearing housing using stand-off spacers. For different diameters of housing, the reaction plate has a series of links that pivot radially to accommodate intermediate diameters, into which the stand off spacers may be mounted.

The equipment may be used to extrrct bushes, liners and bearings from bores from 45mm to 180mm in diameter and up to 180mm long. It can remove both axle liners from a Boeing 747 nose axle in 11 minutes, in the hanger, on the pan or in the workshop.

In the event of an installed bush having a rear access gap of less than the 2mm width of the element nib, these can be ground down using a specially provided holder. This also applies to bushes having very thin walls. The collet elements are relatively expensive and easily replaced.

While the product has been designed for maintenance in the aircraft industry, it is suitable for any situation there the process of removing bushes, liners and metal bearings without damaging either them or their housings is an issue. Further development is envisaged with smaller and larger versions as its use widens into other branches of engineering. A unique feature is that because it is formed from a chain of elements, it can be used to extract objects that are other shapes than circular.

Author
Tom Shelley

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