How Taper Bushing Designs Simplify Shaft-Mounted Drive Serviceability
Shaft-mounted speed reducer technology has not changed much over the past 10 years. Today's drives are connected to shafts by one of several methods: twin-taper bushings; single-taper bushings; or straight-bore designs.
While these designs have proven adequate enough to keep shaft-mounted drives in place, design flaws limit serviceability of the units.
By contrast, a new "torque-assist" taper bushing changes past misconceptions about difficult shaft-mounted drive removal, and begs for a comparison with past designs.
Why Removal Is So Difficult
It is an accepted fact: all shaft-mounted drives and driven equipment will eventually need repairs or upgrading. However, fretted, corroded, or frozen shaft connections can greatly complicate removal.
Single- and twin-taper bushings have been specially designed to relieve these problems by:
- providing a positive shaft lock,
- accommodating greater variances in shaft
tolerances,
- minimizing unit wobble,
- and allowing for easier drive removal.
However, even traditional taper bushings have been known to seize onto the driven shaft over time. The result? Bushings often have to be hammered off of equipment, which can cause drive damage and additional downtime.
Of the basic shaft-mounted drive designs on the market today, each takes a noticeably different approach in dealing with installation and removal problems.
Twin-Taper Bushings
Twin-taper bushings are one of the original designs that address the problem of frozen shaft-mounted drives. Tapers are positioned in an opposed pair configuration to reduce fretting and allow for easier, removal of the drive from the shaft (see Figure 1).
Installation kits include two bushings, keys, mounting fasteners and washers. Inboard and outboard bushings must be aligned with the shaft and unit keyways. Bushing fasteners are alternately tightened (like lugs on an automobile wheel) into threaded holes in the reducer's backup plate; then torqued to specifications (if unevenly torqued, binding and wobbling can occur).
While installation may seem simple enough, removal can pose several problems.
In many cases, the threaded removal holes in the flange can become badly corroded and strip out, requiring the bushing to be broken with a sledge hammer.
The opposed twin tapers must be moved along the shaft out of the unit taper, a maneuver which often proves difficult.
The sheer number of bolts and washers make the unit time-consuming and cumbersome to install/remove--especially in cramped quarters.
Extra axial clearance for removal fasteners and longer driven shaft extensions are required, making upgrade difficult.
And finally, because of the inherent axial clearance problem, the unit and belt sheaves cannot be mounted closely to the driven equipment, which places greater loads on the head shaft.
Despite these difficulties, the twin taper bushing design earned its acclaim by providing the first method for removing shaft-mounted speed reducers.
Single-Taper Bushings
There are a variety of shaft-mounted drives with single- taper bushings available on the market today, also designed to allow easy service and removal (see Figure 2).
Installation requires placing the bushing onto the shaft and aligning the unit bore keyway. Fasteners are then installed through drilled holes in the bushing flange into the unit's backup plate and tightened to the recommended torque.
Just as with twin-taper designs, if the bushing is not evenly torqued, the unit can bind and wobble during operation. Over time, removal holes can corrode and strip out; or worse yet, its inboard flange can break, eliminating the possibility of removing the drive through conventional procedures.
Non-capped, single-taper designs also provide little protection for bushing bores and low speed seals from contaminants. And like its twin-taper predecessor, the single-taper bushing requires axial clearance to allow for the removal of fasteners. Again, this extra distance can place higher loads on the head shaft.
Yet, single-taper bushing designs are popular for several reasons. Single taper designs involve fewer parts for installation than twin-taper bushing designs. As a result, they can be installed faster than other designs.
Straight-Bore Designs
Straight-bore designs with special anti-seize coatings are a third and somewhat obsolete method of shaft drive connection (see Figure 3). A low "up front" cost makes straight bore designs a popular option.
But, quite simply, coatings alone have proven somewhat ineffective against fretting corrosion. Solid bonded coatings will wear away with the working action between the hollow and driven shafts. Anti-seizing compounds eventually dry up or flow away from the area, allowing fretting corrosion to begin.
In addition, straight-bores do not allow for driven shaft variations. If the shaft is undersized, the connection is loose and the hollow shaft can wear excessively. By contrast, tapered bushings compress on the driven shaft even when it is slightly undersized, ensuring a tight connection.
Torque-Assisted Bushings
A newer method of shaft-mounted drive installation and removal is a hybrid of the single-taper bushing design: the torque-assisted single-taper bushing. These taper bushings utilize the speed reducer's torque to power the drive off the shaft bushing during removal.
Installation requires only a few steps. A threaded bushing nut, secured to the taper with a snap ring, is tightened until it draws the unit's tapered hollow bore onto the bushing. This tightening creates a concentric axial force on the tapered bushing, allowing for a wobble-free shaft lock. The set screw is tightened and installation is complete.
The true difference between torque-assisted bushings and others is apparent in the removal process. The bushing nut is locked in position; the v-belt drive is removed; the high-speed shaft is rotated to develop torque which is multiplied by the gear ratio; and a high axial force is generated to free the unit from the bushing taper.
Because such a design does not require extra removal clearance, the drive can be mounted closely to equipment, reducing head shaft loads. By the same token, the v-belt can be positioned closer to the unit, which extends the life of high speed bearings. In addition, the low speed, threaded joint is protected from any contaminants by the large bushing nut, helping to extend the drive's serviceability.
Upgrading Decisions
Eventually, almost all plant maintenance engineers and technicians will be faced with the decision whether to stay with present shaft-mounted drive designs or to upgrade. While problems associated with earlier removal technologies are not insurmountable, some applications, especially those that offer little access, may demand a different approach.
The most efficient shaft-mounted drives will be
those that provide reliability in a design that can be quickly and easily
installed and removed, without the possibility of equipment damage.
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