Retaining: the need for knowledge
May 1st 2008

Bob Orme, senior technology specialist at Henkel, maker of Loctite brand adhesives, discusses the benefits of anaerobic engineering adhesives for cylindrical parts

Traditional ways of retaining cylindrical components include pins, keyways, splines, circlips, grub screws, and press or shrink fits.

However, under some design considerations, these are liable to failure by fretting, and because of the accuracy demanded in producing these parts, they often require time-consuming machining operations.

An alternative solution is offered by engineering adhesives which enable lighter parts with smaller sections to be used. Anaerobic adhesives remain in liquid form while in the presence of oxygen. As a result, the adhesive flows to fill the interstitial spaces between male and female parts. Within this environment of no air and metal-to-metal contact, the curing can take place. Any anaerobic adhesive left outside the joint will not harden and can be readily wiped away.

Most engineering adhesives will generally gain handling strength within 10-20min and full strength in 4-24h, depending on gap, material and temperature. Once cured, the adhesive forms a tough thermoset plastic that, in most cases, can withstand temperatures to 150°C. Beyond those 'conventional' adhesives, there are some specially developed products that will withstand up to 220°C. What's more, all anaerobics are resistant to most chemicals.

With that brief understanding of anaerobic retaining, it's useful to consider some practical examples to show the practice actually works.

Retaining adhesives are used for a range of industrial applications including bushes and bearings, gears/pulleys to shafts, supplementing splines and keys and tubular assemblies. Since the anaerobic adhesive can be used for standard slip fits, many production processes can be simplified in order to remove circlips. In addition, it is possible to reduce stress concentrations by eliminating keys and cross pins.

Calculating the need One example of the successful use of a retaining adhesive features the bonding of a gear to a steel shaft. This particular component is used for high loading requirements in the oil and mining industry. Previously, an interference fit with a key to transmit the torque was used. However, wear of shafts due to fretting corrosion meant machining was necessary on a regular, and therefore costly, basis. Furthermore, when inspections revealed that damage was acute, the gear had to be removed and scrapped – an expensive option.

Using the computer-based program known as Loctite Retcalc, Henkel engineers determined the optimum joint design and type of adhesive needed to overcome the problem. By entering data for static axial pushout force (5380 kN), dynamic axial load capacity (646 kN), static torsional strength (888KNm) and dynamic torque capacity (266KNm), the program revealed a slip fit secured with a high strength retaining adhesive would overcome the difficulties. The solution was adopted, with resultant savings in both time and money. Just as important was the improved reliability for the gearboxes.

Another application involved a manufacturer of petrol pump housings.

This particular housing is used within the fuel dispenser to control the volume of the fuel being pumped in service stations.

The previous method of attaching a sleeve by means of a press fit proved unworkable. Whatever technique was selected for the assembly, it had to offer resistance against petrol and diesel, sealing to 6bar and the ability to withstand temperatures from -30°C to +70°C. When engineers looked into the possibility of using a retaining adhesive, they found a ready solution. Using a rotaspray dispensing device and a medium strength anaerobic it has been possible to incorporate a stress-free joint that meets all the requirements of the components.

Maintenance More generally, unserviceable worn parts can often be reliably repaired with anaerobic adhesives. For example, loose bearings can be refitted in housings - allowing worn housings or shafts to be salvaged. To ensure correct alignment, centring may be required when gaps are big. In many cases, a component can be saved through a combination of the use of metal shims and retaining adhesive.

In the past, adhesives were often considered only when a problem had occurred. Today, an increasing number of production engineers are using them as the method of choice when it comes to retaining.

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