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Bearings - 2014-09-30

Spindle bearings in machine centre

NSK Case Study for the optimisation of spindle bearings

A car manufacturer was looking for a solution to prolong the service life of spindle bearings in their machining centres. Following a thorough analysis of the situation, the problem was solved by using sealed high-precision bearings: The service life of the bearings has now more than tripled.

The bearings of spindles in modern machining centres are subject to high stresses, yet at the same time they are required to provide maximum running precision. This is necessary to produce high quality workpieces even with high axial and radial loads and production based dimensional variances. 

High demands on precision and service life
This is why the bearings have to absorb high radial and axial forces and still guarantee a high rigidity. What’s more, machine reliability is a key requirement in the automotive industry and the bearing plays an important role in achieving this. This is particularly true of machining centres, which are often integrated in fully automated lines, common in the automotive industry. An unscheduled machine failure here leads to costly and time-consuming rescheduling that has several consequences for the productivity of the entire line.

Initial situation: unscheduled failures of spindle bearings
A car manufacturer was experiencing early failures of spindle bearings in his machining centres. The average service life was only 12 months and this caused major problems for the plant management. The manufacturer contacted the bearing experts at NSK who have developed the ‘Added Value-Programme’ for instances such as this. The objective of this programme is to increase the profitability and reliability of customer machines and systems by reducing bearing-induced failures and subsequence lost production to a minimum.

Contamination - a cause of bearing failure
The AIP programme follows a set procedure that begins with a thorough analysis of the actual situation. In this case, the ambient conditions as well as the bearings themselves were investigated. It was discovered that the premature failures of the high-precision angular-contact ball bearings were caused by external contamination.

Recommendation: sealed precision bearings
NSK’s recommendation for the car manufacturer was to use sealed bearings. NSK offers bearings with non-contact seals for this application. On the one hand, this type of seal guarantees a longer service life for the bearings by reducing the wear on the raceways and surface of the balls and ensuring a uniform lubrication. On the other hand, it does not inhibit the rotational speed of the bearings or introduce any friction. This is an important aspect, particularly for very fast-running spindle bearings.

The recommendation was implemented as a trial on one machining centre. The AIP programme also produced further information that showed the contamination had been introduced into the bearing during assembly. NSK therefore trained the customer engineers in the correct handling and installation of precision bearings. Furthermore the first sealed bearings were installed under the technical supervision of NSK with the customer.

Around €50,000 in savings
The results of this modification were very successful, after 36 months, the high-precision, sealed angular-contact ball bearings were still in use. This meant that their service life had been tripled, and because there were still no signs of any failure, an even longer service life is to be expected. It therefore comes as no surprise to learn that the car manufacturer decided to modify all spindle bearings in this department successively – a total of eight spindle bearings each in 15 machines.

The AIP programme always includes a cost appraisal of the recommended measures. In this case, the savings were around 50,000 euros, due mainly to the fact that the bearings gave extended service life reducing maintenance requirements and improving productivity.

Not just for machining
The AIP programme brings some big advantages for not only machining systems in the automotive industry. Ingress of contamination is the cause of premature failures in many industrial applications. A further example proves that AIP can be applied in many situations.

Time and again, an international beverages company had experienced failures in the housed bearings of their conveyor systems. The NSK analysis showed that frequent cleaning of the plant with water and chemicals led to damage of the bearing seals and wash out of the grease lubrication – with the result that the bearings had to be replaced every five to six months and greased every three weeks. The replacement of the bearing units by housed bearings with Molded-Oil technology as suggested by NSK solved this. In this type of bearing, the lubricant is incorporated in a solid polymer matrix which in combination with seal prevents both the penetration of contamination and assures a regular, automatic relubrication. The reduced maintenance (no replacement, no relubrication) saved the operator €5,080 for each plant per year.

App determines the potential savings
Of course, every application case for the AIP programme has to be considered individually. Nevertheless, there are some recurrent problems that allow a sharing of best practice. NSK has capitalised on this and developed an app that is now available. A cost savings calculator determines the total running costs of the actual situation (including service work, downtime etc.) and compares these to the expected overall costs following introduction of the Added Value Programme tools. This makes the potential savings that can be expected very transparent and the user can assess the benefits of introducing the AIP programme.

Sealed, High-Precision Angular-Contact Ball Bearings Have Proven Themselves Under These Conditions.

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