Marland Clutch Backstops Guard Boddington Conveyors
Conveyor systems are increasingly being designed to operate on steeper grades, at faster transfer rates, and with higher drive-power levels to satisfy rising mine and plant production targets. With ore being transported up inclines that can easily span eight stories in some plant installations, a reliable belt backstopping system is crucial to prevent a loaded belt from racing backwards out of control in the event of an unplanned power outage or mechanical failure in the drive.
Conveyor Engineering, Inc. (CEI), a designer and supplier of bulk material handling systems for mining facilities worldwide, needed just such a system to secure an overland conveyor they were building for a mine expansion project.
The Boddington gold mine (BGM) is located 128 km (80 mi.) southeast of Perth in Western Australia. The original oxide gold mine ceased operations in late 2001. Denver, Colorado-based Newmont Mining Corp., which previously owned two-thirds interest in the mine along with AngloGold Ashanti and is set to become full owner when a buyout deal with AngloGold is finalized, authorized an expansion project in early 2006 aimed at developing, mining and processing gold and copper ore which lie beneath the depleted oxide pits. The mine, scheduled to start up this year, will produce an average of 1 million oz/y gold and 30,000 mt/y copper at a processing rate of approximately 35 million tons per year and a mining rate of 80 million tons per year.
To meet the required capacity, CEI designed an 1800 mm x 2.2 km (70.9 in. x 1.4 mile) belt conveyor system driven by two 2800-kW (3,750-hp) motors to handle 8,000 mt/h of ore. The conveyor runs at a belt speed of 4.5 m/s (885 ft/min), or a head shaft speed of 53 rpm, up a 15-degree incline.
The specification for this conveyor system required a guaranteed backstopping holding torque of 975600 Nm (720,000 lb-ft.). Adding to the challenge, the backstop will operate in an environment of airborne grit, in temperatures that can reach 46°C (115°F), and is expected to run 24/7/365 for 20 years.
CEI specified Marland Clutch Model BC backstops for the project. Working through its sister company, Warner Electric (Australia) Pty. Ltd., Idaho, USA-based Marland Clutch, a division of Altra Industrial Motion, provided 21 backstop units for the BGM project. The largest units supplied were BC-720MA backstops, having a shipping weight of 4500 kg (10,000 lb) each and a bore range of up to 533 mm (21 in.). The shaft diameter where the holdbacks are mounted is 430 mm (16.9 in.).
Unlike electrical or pneumatic clutches and brakes, these ramp-and-roller style backstops are completely mechanical and automatically engage with any attempt at reverse rotation. Since no controls or adjustments are required to operate the backstop, it provides high reliability at a lower installed cost.
The Marland units are mounted at the point of highest safety, immediately adjacent to the drive pulley. “That means the backstop does not have to rely on any other component in the drive train to do its job,” said Dave Stoltze, Marland Clutch chief engineer. “A problem with any part in the belt drive system will not inhibit the backstop’s ability to reliably hold the load.”
The clutch elements and tapered roller bearings are continuously self-lubricated in a sealed oil chamber. The backstop’s grease labyrinth seal design prevents airborne grit from attacking internal oil lip seals, which can cause them to leak and eventually wear out if left unchecked.
Marland says its backstops have replaced competitive brands that failed in similar applications, including one recent end user who required full-load testing prior to purchase. Marland successfully performed full torque load testing at 508000 Nm (375,000 lb-ft.) on its BC-375MA model, as well as standard freewheel testing at maximum catalog speeds, checking both temperature and vibration. Since the BGM installation required Marland’s largest backstops, with nearly double the torque capacity, the company decided to test the BC-720MA units as well. A 50-ton hydraulic jack was used to apply a torque load of more than 975600 Nm (720,000 lb-ft.), and the units performed as expected. All 21 backstops were shipped via ocean freight and delivered to the worksite ahead of schedule.
Marland says it typically can deliver backstops up to 381 mm (15 in.) in bore diameter within four weeks.
According to the company, conveyor equipment consulting firms have indicated that the next generation of conveyors will be larger and will require even bigger backstops. To meet these needs, Marland developed designs for two new backstop sizes, one of which will have a catalog rating in excess of 1.36 million Nm (1 million lb-ft). New torque arm options will include load sharing, torque limiting and fully releasable designs. These optional systems allow multiple backstops to more closely share reversing torque to further extend capacity.
Incorporating Hydraulics To Properly Mount and Dismount Bearings
By Greg Hewitt
Properly installing large bore roller bearings is essential in achieving the maximum life of a mounted bearing. It is estimated that the second most common cause of failure of large bore bearings is improper mounting techniques. Failure can come in the form of shaft attachment loss, excess vibration, and elevated bearing temperatures. Following proper mounting techniques will ensure that the bearing is mounted correctly and that the maximum life of the bearing will be realized.
There are many different methods of mounting large bore bearings. Common methods include manual assembly, heat shrink, jack screws, oil injection, and hydraulics. The most common is the manual method incorporating the use of a hammer and drift, or spanner wrench.
The manual approach involves a bearing, plus a sleeve, nut, and washer. The adapter sleeve would be slid over the shaft, and the bearing over the adapter sleeve. The nut would be screwed onto the adapter sleeve, which then forces the bearing up the tapered OD of the sleeve. The movement of the bearing up the tapered OD of the sleeve would in turn reduce the clearance in the bearing and therefore provide a press fit to the shaft.
Shim stock is used during this mounting process to measure the amount of radial clearance removed from the bearing. The shim stock is inserted between the roller and outer ring to determine the clearance reduction. When using this method, the user tightens the nut, checks the clearance reduction, continues to tighten the nut, and continues to measure the clearance reduction until the proper amount of clearance has been removed. This method has always been suspect as human error influences the measured values. It’s also very time consuming, requires extreme physical effort, involves potential safety hazards, and in the end leads to a questionably mounted bearing.
A solution to this problem is to use hydraulics to mount the bearing. The use of hydraulics dramatically decreases mounting time, eliminates much of the manual effort required, reduces the potential for injuries, and ensures a proper press fit between the shaft, adapter, and bearing. When mounting a bearing using hydraulics, the standard nut is temporarily replaced with a hydraulic nut (Figure 1).
The hydraulic nut consists of two pieces; the nut and a piston. In this scenario the hydraulic nut is screwed onto the tapered adapter sleeve. Fluid is then pumped into the nut causing the piston to extend. The piston comes into contact with the bearing inner ring and pushes it up the tapered adapter sleeve to the starting position. The starting position is considered the point at which the clearance between the shaft, adapter sleeve, and bearing bore has been reduced to zero and the bearing is snug to the shaft.
The next step is to place a measuring device onto the bearing inner ring or face of the piston to measure axial displacement. The measuring device can be as simple as a magnetic base indicator or a displacement gage. Continuing to supply hydraulic pressure to the nut will further displace the piston until the final position is obtained. At this point the hydraulic nut would be removed and replaced by the standard nut. The starting position pressure and the required final displacement values are supplied by the hydraulic nut supplier and the bearing manufacturer. Applying hydraulic pressure to set the bearing, and measure axial movement of the bearing to remove the required clearance, replaces the use of shim stock and therefore provides for a consistently and properly mounted bearing.
Dismounting the bearing can also be a very tedious and time consuming process, especially if the bearing was mounted without a predetermined method to dismount the bearing. There are several methods of removing bearings, including a manual method, the use of oil injection, or a hydraulic dismount nut.
Manually dismounting the bearing usually consists of torching the bearing from the shaft, which destroys the bearing and can damage the shaft. This is a very time consuming and costly method, especially if the bearing had not failed but just needed to be repositioned.
The use of oil injection requires that the shaft, or a special adapter sleeve, contain a hydraulic port plus blind circumferential grooves that allows hydraulic fluid to be pumped in between the bearing ID and the shaft or adapter OD. While this method will not cause damage to the bearing or the shaft, the hydraulic oil that filled the grooves spills out when the bearing suddenly becomes dismounted. However, the biggest disadvantage to this method is the high cost of the specially machined adapter sleeve or shaft (Figure 2).
Recognizing the benefits of incorporating hydraulics to mount and dismount bearings, mounted bearing manufacturers are now supplying products that include preassembled hydraulic mount and dismount nuts (Figure 3).
Unlike conventional hydraulic nuts which are separated from the bearings after assembly, the hydraulic nuts used in these new products are an integral part of the bearing assembly and stay with the bearing for the entirety of its life. The use of the hydraulic dismount nut will remove the bearing from the shaft quickly and easily, without damage to either. Basically a hydraulic dismount nut functions in the same manner as a hydraulic mount nut. Hydraulic fluid is supplied through the nut and to the piston, which causes the piston to extend and come into contact with the inner ring. Increasing the hydraulic pressure will force the bearing down the tapered adapter sleeve until the bearing is dismounted.
Correctly mounting large bore roller bearings is a critical step to achieving optimum bearing performance and maximum bearing life. By selecting a bearing with a hydraulic system already built in the product, you will reduce mounting and dismounting time, eliminate bearing and shaft damage, and most importantly, end up with a properly mounted bearing.
Hewitt is a Dodge bearing development engineer for Baldor Electric Co.