By Grant Laidlaw
I have found that many technicians do not understand how to fit or remove a taper bush.
Andre asks: Grant, Twe have a special problem – when working on units with pulleys or couplings we find that the taper-lock bushes are always broken in half, any idea as to why? Thanks.
Hi Andre, yes this is an interesting question, In most cases it is not necessary to use a hammer or any excessive force to remove a taper bush. In doing so, one causes the damage which you are referring to. In addition incorrect installation could result in pulleys wobbling, vibration, excessive wear of components, or in fact – a catastrophic failure of the taper bush.
A damaged taper bush. Image supplied
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The taper lock bush, also referred to as a taper bush or taper fit bush, is a locking mechanism commonly used in power transmission drives for locating pulleys, sprockets, and couplings to shafts.
Pulley with taper bush and grub screws. Image supplied
The taper lock bush is pre-bored and keyed to match the required shaft and keyway diameters.
This allows tight, concentric clamping of a pulley or coupling onto a shaft, and subsequent relatively easy removal when necessary. The system is highly reliable; such failures as may occur are mainly due to misapplication or incorrect procedures.
Let us have a look at the concept.
The bush which clamps onto the shaft has an external taper of 8º. This fits into a hole in the hub of the pulley/coupling which has exactly the same 8º taper.
Grub screws are used for fastening and removing the bush fit into holes drilled half in the bush and half in the hub. One half hole is drilled right through, and has a thread tapping. The other half hole is drilled part way, and offers a clearance to the screw.
It is very important to note that the holes used for fastening the bush have their threads in the hub of the play or coupling. Do not use the other holes as this can damage or cause misalignment to the taper bush.
When fitting a taper bush the allen screws are tightened pushing down on the end of the hole which is not through-drilled, driving the bush deeper into the bore of the hub. As the hub bore is tapered, and as there is a ra dial split with clearance in the bush, this action results in the bore of the bush being squeezed tighter, with very great pressure. According to the size of the bush, there are either two or three holes where this tightening is done. Gradually tightening the grub screws in a sequence ensures that the bush is pulled up uniformly.
The removal procedure involves removal and transferring of the grub screw(s) into the removal hole(s). When a grub screw is transferred to this hole(s), and is tightened, the forces are reversed, instead of pulling the taper bush into the taper the bush is pulled out of the taper. As this happens, the taper bush can expand, loosening the fit around the shaft. The taper bush should slide off the shaft with relative ease.
Fitment and removal of a taper bush. Image supplied
The smaller bushes are supplied with three or four holes, and the larger with five. Note: in some instances the fourth hole of the smaller bush, which is located so as to run into the split, is there for providing balance only. You therefore should never use this hole, either for fastening or for removal of the bush.
In the smaller size of taper bushes, two holes are used for fastening and one for removal and in the larger taper bushes, three holes are used for fastening, and two are used for removing the bush.
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Note that it is common that the grub screws are BS Whitworth. If screws get lost, obtain a proper replacement from the supplier.
The entire taper lock bush and the bore of the hub are supplied with a protective anti-rust coating. This must be removed as the safe and secure operation depends on metal to metal contact, without any grease or oil remaining behind on the surfaces.
Use a solvent to clean up the components.
It is good practice that a taper lock bush should always be fitted with a key.
With regards to the the fitting of a parallel key; the key must be an interference fit in the sides of the keyway, it must have a transition fit in the sides of the bush keyway and a clearance fit at the top of the keyway.
If a parallel key, which offers no top clearance is fitted, the bush will ride on this, and will not seat correctly on the shaft. Stresses will be set up, with stress concentrations at the corners of the keyway. In time, if not immediately, cracks will appear. If left, these will progress more and more deeply into the bush. Any bush which has any sign of cracking must be discarded.
Inspect the shaft and key carefully for burrs, and remove if necessary, to ensure that the bush will go onto the shaft without obstruction. Having checked the key size regarding the required top clearance and transition fit at sides of the bush keyway; fit the key to the shaft keyway. Here it should be an interference fit.
Installation
When installing the taper bush, line up the holes in the bush and the hub, and insert the bush.
Place a small drop of oil on the treads of the grub screws, and insert these into the holes which are threaded in the hub, remembering that it is important not to get any oil at all onto the surfaces of the taper bush itself. Turn the grub screws loosely into place.
Slide the assembly onto the shaft. Note that the bush will hold its place on the shaft where you position it, as it is being tightened. The hub will pull along the bush as you pull up the grub screw. Bear this in mind for alignment purposes.
Do not attempt to open the bush by driving a screwdriver into the split section. This will, almost without doubt, break the bush, rendering it useless. If there is difficulty in getting the bush to slide onto the shaft, this should be investigated and resolved. Be sure that all burrs have been removed. Check that you are not trying to fit an inch size taper lock onto a metric shaft, or the other way around.
In this regard, there is a possible problem between 1 inch and 25mm as the two are very close. They are however not interchangeable, 1 inch equals 25,4mm.
With an allen key continue to gradually and equally tighten the grub screws, making sure that the taper bush remains in the intended position, alternate between the two (or three) screws, making sure to never get any one too far ahead.
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Note that the grub screws may be imperial; you will need to use an imperial allen key.
Using a nylon hammer, tap on the larger end of the bush. This will ensure concentric seating by the bush. After doing this, further tighten the screws. (You will find that they are now able to move more). Repeat the tapping and tightening, until there is no more movement to be found on the screws.
You can torque the grub screws to the manufacturer’s published values given for the tightening of the grub screws. The tables, carrying this information, are available in the manufacturer’s literature.
Allow the drive to run under load for a short time. Then stop the system and recheck the tightness of the grub screws. Once complete you can apply a small amount of grease to the grub screw heads.
Do not grease or apply other coatings to the bush or the shaft at the bush in order to prevent rusting. There will be no rusting of the surfaces where metal to metal contact is being made.
Andre, let us move on to the removal of the taper bush.
Removal
To remove a taper bush, first loosen off all the grub screws. Apply commercial penetrating oil if rusting has taken place, after first having cleaned the shaft end where the bush will be required to slide.
Oil the threads. Fit one or two of these screws (according to the design of the taper bush) in the jacking out-hole or holes.
Tighten the screw(s) – in the case of two screws alternately – continuing until the hub and assembly draw free on the shaft. You can now remove the taper bush with ease and without damage.
Thank you for the question Andre, I hope that this helps with your work with taper bushes.
References:
- ACRA
- SANS 10147
- Fenner
About Grant Laidlaw
Grant Laidlaw is currently the owner of the Air Conditioning and Refrigeration Academy (ACRA) in Edenvale. He holds a Bachelor of Business Administration and an associate degree in educational administration. He has a National Technical Diploma and completed an apprenticeship with Transnet. He has dual-trades status: refrigeration and electrical. He has been involved with SAIRAC for over two decades and served on the Johannesburg committee as chairman and was also president between 2015 and 2018. Currently he is the SAIRAC national treasurer.
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