Tuesday, April 28, 2015

Coupling Failures on Steel Mill Tables

The failure

A leading Midwest steel mill plant was constantly replacing tire couplings on runout tables. Needless to say, the amount of time required for maintenance was becoming unmanageable.

The equipment and environment

This particular hot strip mill plant produces a coiled steel rolls commonly used to form large kitchen appliances and automobiles.  Runout tables, containing hundredths of rollers, move steel slabs along the shaping process. Each roller uses a tire coupling that attached to a motor, which turns the roll.

High misalignment and a lower service factor between motor and roll can cause high failure rates on the tire couplings.  In addition, each tire coupling has several component fasteners that require tools to change the element. The design of the tire coupling makes it susceptible to corrosion from the heat, water, and slag associated with a runout table application.

The steel mill plant had a dedicated maintenance team to replace the large quantities of tire couplings every month. The high replacement part cost, increased maintenance time and resources required to maintain the tire couplings was prohibitive.

The solution

After a fact-finding visit to the hot strip mill plant, the Lovejoy Engineering Team analyzed the situation and presented an effective solution.

The team addressed three core runout table failures:
1) Tire coupling slipping
2) Heat, water, and slag damages
3) Inefficient inspection process.

The recommended solution was to replace all tire couplings with the Lovejoy’s Jaw In-Shear 6 Pin (JIS) couplings.  The Jaw In-Shear 6 Pin coupling is made up of a standard L-type jaw coupling, a radially assembled element, and stainless steel locking ring.

With its effective locking mechanism, the JIS coupling eliminated the slipping issue.  The stainless steel locking ring provided the element with protection from heat, water, and slag. Lastly, the ease of simple visual inspection and quick change-out, made the JIS the most effective solution.

In the end, failures were reduced to a minimum.  The dedicated plant maintenance team could now refocus its attention on other critical areas in the mill.  Furthermore, this solution brought in a 33% cost savings per coupling.

Monday, April 20, 2015

Coal Fired Power Plant Pulverizer Gear Coupling Failure

The Challenge

A plant manager, from a leading coal-fired plant located in the Midwestern United States, approached us with a question about a gear coupling failure in its coal pulverizer drives. 

Power Generation Industry background

More than 50 million consumers and business owners from Ohio to New York and Canada would agree that a consistent power supply is not only vital to the overall economy, but also for the comfort and stability of daily life. The major power outage that struck the U.S. and Canada in mid-August of 2003 cost New Yorkers alone, an estimated $500 million. Although experts point to a dated power grid as the main cause of the recent outage, every detail of the power generation system, including the  smallest component, plays a critical role in maintaining the electric power plant.

Equipment and Components

Depending on the size, a single plant can have more than 25 coal pulverizers. Coal pulverizing allows for less emissions and more efficient use of the fuel. The coal burns to create the steam that turns the generators and provides electric power. Large electric motors use a worm gear arrangement to drive the coal pulverizers and a blower that sends the pulverized coal to the furnaces. The shafts of the motor must be mechanically connected to the worm gear and blower. The worm gear is a relatively small, yet vital component. If it fails, the pulverizers and blowers can no longer operate.

Dissecting the problem

Gear couplings allow for axial movement of the shafts, which increases the wear on the gears, sleeve bearings and other components. This wear on the components would cause them to fail prematurely.
The loss of production due to downtime was becoming an issue. Bearing replacement costs were in the thousands of dollars per bearing.

Preventative maintenance costs associated with gear couplings were also becoming a burden due to time spent on inspection and lubrication to keep them operative. The power plant’s rigorous preventative maintenance program required regular disassembly of the gear couplings for inspection, cleaning and re-lubrication of the gear teeth. In addition, assessment of the condition of the couplings was a judgement call unless extensive and time-consuming measurements were performed. Coupling replacements also require heavy machinery to move the motors.

Old technology, old solution

The existing gear coupling design was typical of the technology available when the plant was built in the early 1970’s. Where gear couplings did not limit axial movement of the motor shaft, sleeve bearings were used. This old technology configuration increased wear and shortened life spans of both the motor sleeve bearings and the gear couplings.

In other words, increased sliding velocity of the gear teeth in the gear coupling creates a lot of heat and wear, just picture yourself rubbing two sticks together. The motor moves a bit, the gear on the coupling moves a bit, and this motion could be repeated up to a million times over the course of the day, contributing to premature failure.

Contemplating a similar solution

Install gear couplings with limited end float; however, its maintenance nightmare will continue because the gear couplings are difficult to assess for wear and the inspection and replacement of the gear coupling still required removing the motors. The gear couplings would still have to be disassembled, inspected, cleaned, and re-greased every time; definitely not an effective solution.

New Technology, new solution

Install a Disc Coupling. Unlike gear couplings, the Lovejoy SX Disc Coupling design is inherently balanced. There is less vibration than the gear coupling as it wears. The disc coupling performs a limited end float function without special design considerations for the sleeve bearings. Maintenance-free operation is achieve because the Lovejoy coupling has no moving parts and requires no lubrication. This translates into a long coupling life.
Actual Pulverizer Solution

Preventative maintenance and inspections of the disc coupling’s flexible elements was now possible using a simple strobe light and while the coal pulverizer is running. Hence, there is no need for an expensive shutdown and disassembly. Replacement of the disc packs became a breeze without moving any machinery or disturbing the original alignment.

Bonus features

Lovejoy was able to provide a heavier duty coupling than the gear type in the same space as before. Nothing had to be moved or modified for installation. Additionally the component cost was comparable to those of a gear coupling. The Lovejoy’s design allows for more capacity and misalignment capability than any other disc coupling available today.

In addition, disc coupling signals the maintenance staff when it needs replacement – you can actually see it with your own eyes without any disassembly. In contrast, in gear coupling is more of judgment call, unless you take it completely off the machine and measure it extensively.  With the Lovejoy disc coupling, the outer leaf of the disc will crack before the entire disc pack fails, signaling to the maintenance staff it needs replacing.

Happy conclusion

The coal pulverizer is critical equipment, if it is not blowing coal, it is not generating power; needless to say, would you rather have your pulverizer blow coal or blow off your money?. The disc coupling solution reduced the maintenance nightmare, and the plant has been experiencing success for more than a year with the original installation. Plant management approved the proposal to convert all gear couplings on 24 coal pulverizers to Lovejoy disc couplings, based upon the proven benefits and demonstrated cost savings.

Monday, April 13, 2015

Coupling Solutions Learning Lab - Hannover Messe 2015

Coupling Learning Lab at Hannover Messe 2015
Coupling Learning Lab - Hannover Messe (Hall 25, D34)
If you've ever been to a major trade show, you will notice that most manufacturers go to great lengths to highlight only their absolute best looking products. With this in mind, we thought we would take a radical approach, and create a booth filled with coupling failures for Hannover Messe 2015 (Hall 25, Stand D34).  

That's right, a trade show booth filled with all kinds of product failures. 
The natural question one might expect is: "Why?" (And sure enough, before the show has even started, one competitor walked up to our booth and said "Why are you showing bad couplings?")

Failed Disc Couplings at Hannover Messe
Failed Disc Couplings at Hannover Messe
The answer is simple. We genuinely care about our customer and want to educated them as to what may and can go wrong if they do not size, select, or install a coupling correctly. Having been a market leader in the flexible coupling market for generations, we are also more than confident enough in ourselves and our products to not worry about the impact of highlighting products that were not used correctly. 

By design, couplings are generally a wear item with a finite life. Furthermore, they are designed to take the excessive abuse of a system, and are often designed to formally serve as a system fuse. 

R+L Hydraulics Booth at Hannover Messe 2015
Primary Hannover Messe 2015 Booth (Hall 20, C36)
If you are lucky enough to be in Hannover, Germany and attending Hannover Messe 2015, we hope you will stop by our Learning Lab to ask whatever coupling question you may have, learn from what we have on display, and are able to grow from the incredibly talented engineers we have staffed the booth with this year. 

Will this approach work? While we are quite optimistic, we of course would love your feedback. Please let us know if you think the booth was a good idea, and how we could improve upon it in the future. 

And, if you really can't stand the idea of seeing worn out couplings... or just would like to be reassured by a whole bunch of those as well... we'll we still have you covered at Hannover Messe. In addition to the Coupling Learning Lab booth, we also have our primary R+L Hydraulics hydraulic & power transmission booth in Hall 20, Stand C36.

Thursday, April 9, 2015

When Breaking Up is a Good Thing

A customer recently called in to Lovejoy concerned about a jaw coupling failure. The unusual aspect of this issue was that three hubs broke in half in one week (see picture 1). Lovejoy’s technical team was baffled by this inquiry and began the process of analyzing the failures.

Picture 1.
This particular application had an electric motor (30 HP) driving a blower at 3510 RPM.  Initially, our engineers focused their attention on over torque. The jaw coupling used in this application had an NBR spider rated for 792 in-lbs. of torque. The application had a torque rating of 650 in-lbs. Based on the torque specs alone, one could say that the couplings should have performed well under normal conditions (for more details on calculating torque see Coupling Sizing Torque - How to Quickly Calculate).

In conjunction with the over torque troubleshooting, the end user confirmed that the alignment was within the coupling specifications. The customer used his own methods and tools to verify alignment, and he also followed the Lovejoy Install Videos and Install Guide step-by-step procedures.

As our technical team continued with the troubleshooting steps, they took the time to share additional technical details as to what can cause torque spikes.  One of possible causes is motor pauses and restarts.  The customer then mentioned that they had an electrician replace a fuse on the motor because it had blown out previously. Furthermore, the customer mentioned that there were momentary pauses that occurred after replacing the fuse, which is an indication that the motor was causing torque spikes.
Jaw Coupling Hub - Signature Keyway Burst Overload Failure
Picture 2.

Although fail-safe by design, jaw couplings can fail catastrophically when hit with a large enough over-torque. (This would be a good time to remind you that OSHA requires coupling guards for all flexible coupling applications. Please be safe.)  When a jaw coupling hub fails catastrophically, it will typically fail either by shearing at the jaw legs, or by breaking open between the inside bore and outside diameter of the hub (generally along the keyway - see picture 2).

In this particular case and because of torque rating specs, it turned out the coupling breaking in half was actually a good thing.  If the couplings were to continue to transfer torque between the motor and the blower, the probability of burning the motor or damaging the bearings in the blower becomes high.   Breaking up (coupling failure) allows you to spend a few dollars to fix the problem as opposed to thousands of dollars in equipment replacement.

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