With todays challenges, it is critical to increase efficiency in manufacturing processes to be profitable and sustain profits. A disciplined machine reliability approach involving OEM and vendor personnel can ensure success.
In this article, a case study involving a large steel mill will be used. A two-year machine reliability program was initiated, which resulted in a number of intended and unintended cost savings.
What Constitutes a Disciplined Approach to Machine Reliability?
The key elements to a disciplined approach to machine reliability involve Six Sigma-type methodology. They are applicable to all types of manufacturing operations from many industries.
Below are the main elements that would be involved:
1. Identify and quantify high cost and sources of downtime
2. Work as a team
3. Gather facts
4. Investigate and offer solutions
5. Implement solutions
6. Validate solutions – provide OEM with evidence
7. Periodic follow-up
It is generally known where the high cost and problem areas are within a plant. However, it is not generally known if there is a cost-effective solution and, if a solution is implemented, how it can be validated to ensure real savings can be realized.
Identify and Quantify High Cost and Sources of Downtime
Initiating a project that works to provide theoretical savings is generally not advisable. Any machine reliability programs must target and directly hit areas that provide real savings.
Good machine reliability programs target critical assets and the cost of maintaining critical assets. Aspects that reflect real savings are outlined below:
– Having knowledge as to the effect of increasing the mean time between failure and its impact to improve profits should be gathered.
– Reduction in maintenance hours – spend more time on other aspects to improve efficiency
– Cost of replacement components that fail
– Cost of machine downtime of unplanned failure of equipment
– Reduction in lubrication – Overgreasing is common and can be overcome with better lubrication practices; grease costs and disposal are areas that have generated real savings in steel mill applications
In this case study, the OEM was able to quantify the following:
– Bearing replacement quantity and cost
– Roll repair maintenance cost that had less than expected mean time between failure
– Maintenance costs of installing equipment that failed short of scheduled maintenance
– Costs associated with lost production from unplanned downtime
As the project began, major additional cost reduction was realized as to grease usage and electrical energy.
The essence of the machine reliability program that involved high cost centered around rotating equipment associated with bearings, seals and lubrication.
Work as a Team
The next step was to work with a vendor and bring in knowledge that covered the target areas. One specific vendor with expertise in bearings, seals and lubrication participated. To this end, the OEM assigned a competent reliability engineer as a project manager along with key maintenance personnel. The vendor also assigned personnel with competency in bearings, seals and lubrication.
A team comprised of the OEM project manager and three vendor personnel was formed to assist in gathering facts and evidence as to the root cause of failure. A single point of contact was developed to transmit information that could be documented in an orderly fashion.
The team would meet twice a month and report on findings associated with critical assets that were a source of high cost.
The approach taken by the OEM and vendor was to look at rotating equipment as a system – a series of critical mechanisms. These mechanisms included the bearing, bearing housing, shafts, seals and the lubrication system. The team evaluated the rotating mechanism, keeping in mind how one mechanism can support other mechanisms. The key elements in rotating equipment can complement each other or can oppose each other. Because the vendor had bearing, sealing and lubrication experts on the team, solutions were developed that were complementary to each mechanism.
The next step in a good machine reliability project is to gather facts. This is best done as a collaborative effort with the OEM and vendor experts in relevant areas. Facts were gathered as to the root cause of poor efficiency and high costs. Some areas that were relevant in this particular steel mill application were:
1. Certain maintenance practices with high cost
2. Root cause failure analysis of bearing failures
3. Use of technology – anything better since the plant’s design and construction?
4. Sealing effectiveness in rotating machinery
5. Lubrication practices
Results from the investigative work with the OEM were as follows:
Study Maintenance Practices
1. High Cost of Reconditioning Roll Shafts – The roll surface condition at the location of seal contact was far higher than what a seal can tolerate. The roll surface condition evaluation found that a surface roughness of 150 to 200 micro inches was typical. For effective sealing, 16 to 32 micro inches is desired.
2. Bearing Installation – Induction heater did not demagnetize the bearings.
1. The root cause of bearing failures related to high levels of contamination. Depending on the location, contamination water content was 30 percent, and iron-oxide contamination was 20 to 25 percent. All contamination level results were determined by performing FTIR analysis of the grease.
2. Secondary, but less frequent, were bearing failures from misalignment during installation.
Use of Technology
1. The original mill was designed with cylindrical roller bearings (CRB). As the mill structure aged and degraded, misalignment of rolls shafts was common.
2. The old seal technology was still in use. Seal life was shown to be only nine to 12 months.
1. Seals, as installed, were not used in a complementary way to work with the lubrication system.
1. Ineffective lubrication practices were being used as the primary way to flush out contamination.
In all aspects of a disciplined machine reliability program, the vendor must proceed in a way that causes minimal disruption to the OEM. To this end, the vendor had developed tools and techniques so that evidence could be gathered in such a way that it would not disturb the flow of maintenance work. The vendor must be accessible during times when the equipment is being repaired and installed as well as when critical maintenance tasks are performed. Tools used to gather and document evidence are critical to the OEM so that it can be readily evident as to the root cause of the problem and not be just an assumption.
Investigate and Offer Solutions
The next step is to offer solutions. These solutions must have a reasonably good chance of success and ultimately be validated to the customer’s satisfaction. Like the fact-gathering phase, solutions must be offered in such a way that the validation of the solutions can be done with minimal disturbance to the OEM.
In this case study, the vendor prepared a presentation on the findings and solutions, as follows:
1. Training was given on the proper use of an induction heater to demagnetize the bearings.
2. Speedi-Sleeves were used to eliminate the need of roll shaft repair. Speedi-Sleeves could be pressed on easily and provided an ideal surface finish at the location of the seal contact area.
1. New seal technology and chemistry were used and were not cost-prohibitive. A double-lip, wear-resistant seal was proposed.
2. Use of spherical roller bearings (SRB) allowed for improved alignment of the roll shafts to reduce misalignment as the cause of bearing failures. The configuration of the spherical roller bearings was such that no modification to the bearing housing was needed.
Revision to Lubrication Practices
1. Present lubrication practices involved purging of grease through the bearing housing purge hole using a 5 psi relief valve. The location of the purge hole and relief valve did not allow the majority of contamination to flush out of the bearing housing. The lubrication solution was to take advantage of the sealing solution.
The bearing housing purge hole was plugged, and all purging of grease was done through the seal. Purging grease in this location significantly flushed out any ingress of contamination. An added effect was that seal life was increased substantially, as a film of grease under the seal lip reduced wear.
Illustration of Solutions
After collaborative consultation with the vendor and OEM, it was agreed that the following solutions be validated in a trial location in the mill:
The maintenance practices to demagnetize the bearings and install Speedi-Sleeves on the roll shaft were implemented. Four test rolls in two locations were built up with the new seals and bearings. The bearing housing purge hole was plugged, and the lubrication purging was set up to purge through the seal.
The roll torque on spherical roller bearings installed showed to have 7 to 15 foot-pounds less roll torque. This reduction in roll torque, when implemented on all 880 rolls, provided an energy savings of $275,000.
In 18 months, bearing grease samples were analyzed by both FTIR analysis and Hegman gauge. The results showed a reduction in contamination from typical 25 percent water and iron-oxide content to less than 10 percent. Seal evaluation wear showed that with the combination of seal wear-resistant chemistry and purging grease through the seal, seal life would increase from nine to 12 months up to five years. Tests were conducted to measure the reduction in grease purge pressure and compared to lab analysis to arrive at a projected seal life of 60 months. As the seal wears, the purge pressure would be reduced. There was a direct correlation between purge pressure and seal wear.
Validation of Solutions
As the test mill was running, data was gathered as to what improvements were being achieved.
Seal Wear Assessment
A grease gun was used to pump grease into the bearing housing. A pressure gauge was installed at the bearing purge hole. The grease was injected into the bearing housing, and the pressure at which grease purged out the seal was recorded. As the seal wears, purge pressure reduces. A great deal of lab testing was done to coincide with the mill application to confirm the seal wear rate. Several methods of acceleration wear were performed in the test lab. They correlated well with the wear rate at the mill test locations.
Below is a graph of a seal purge pressure trend log:
The chart below illustrates that in more than nine samples of purge pressure testing (approximately one test per month), seal wear produced 1 psi drop in purge pressure over seven months. The known hours of running time also were recorded.
Through extensive seal life testing, a seal life plot was produced. The life of the original single-lip nitrile seal was significantly less than the new seal with the double-lip configuration.
Through use of machine reliability Weibull distributions, a reliability plot of new seal life was provided to the OEM.
Validation of Lubrication Method
With the improved sealing, lubrication in the test locations was gradually reduced. The purging of the grease through the seal was performed once per month for 18 months. Grease samples were taken out of the bearing housing twice per month and analyzed. It was found that the lubrication amount could be reduced 65 percent before adverse levels of ingress contamination occurred. The target was that no level of ingress of contamination be more than 10 percent. The reduction of grease produced a significant cost savings in grease consumption. It also resulted in a reduction of costs associated with grease disposal.
Below are the methods of taking grease samples and documenting the results used to provide evidence to the OEM.
Only a team and disciplined approach can achieve machine reliability that provides real savings. Providing solutions that can be shown to achieve a high probability of success, along with testing and validation, allows the OEM to be assured that meaningful cost reductions can be realized.
This article was previously published in the Reliable Plant 2016 Conference Proceedings.
By Paul Conley, SKF