Pumps & Systems, February 2008
Self-timing, two-piece timing gears have changed the timing procedure and increased the life cycle of twin screw pumps.
Old Technology Versus New Technology
In the past, timing gears were made with hardened gear teeth only. The bore was left soft so that the keyway could be cut to time the pumps. Possible problems included a wallowed out and oversized keyway, which caused the gear to move around, wear the shaft and require repairs. The bore also became oversized, and the gears became unusable for any further service. If the gear stripped the keyway, it quite often gaulded on the shaft and made the gears unusable for a second run. Even with a re-keywayed gear, a large gaping slot takes away from the alignment area of the bore.
In two-piece timing gears, the hardened keyway and bore eliminate these problems. Each gear-one for each rotating shaft-has two components: a splined hub that is bored and keywayed to fit the existing rotating shaft of the pump, and a gear of the same style, tooth type and outside diameter of the original timing gear. This component is bored and internally splined to properly slip onto the spline hub.
The number of splines are determined by the pump size and the clearance required by the individual pump. In the average twin screw pump, the change in clearance between the meshing screws is about .001-in for each .004-in distance in circumference at the pitch diameter of the timing gears. Adjustments as small as at least .004-in control how many splines are used in a set of timing gears.
For example, for the size NF pump with a center distance of 3.375-in, each of the outer gears will have 33 gear teeth with an outside diameter of 3.500, which is standard with twin screw pump manufacturers. Both timing gears have 33 external teeth. One of the timing gears will have 83 splined internal teeth, and the mating timing gear will have 82 splined internal teeth.
When you slide the outer portion of the timing gear off the splines, roll it one tooth and slide it back, one gear advances at a rate of 1/83 and the other gear advances at a ratio of 1/82. It works out to be a difference of about .001-in at the pitch diameter of the gear, which is 4 times closer than limits preset at the beginning. (Advancing both gears one spline tooth in the same direction only changes the position of the outer gear one-thousandth of an inch at the pitch diameter.) This set of timing gears has 5445 possible timing positions, of which 907 are accurate timing positions (see the next section for the timing process).1, 2
Involute splines are the most efficient way to transmit torque. Full fillet 30-deg pressure angle splines are preferable because they are the strongest kind. The splines are class B, which are nonsliding under a load. They resemble gear teeth and are cut on the same machine, but they have no rolling action. The teeth all fit together, and the absence of relative motion means that wear is not a problem. Consequently, the splined hub portion of the gear will see no or very little wear and a new outer gear can be installed onto the old spline hub portion. Pumps equipped with two-piece timing gears make it possible for the user to replace the outer gear portion only if the timing gears need changing a second time.
The position of the keyway in the splined hub portion of the two-piece gears is not critical, since each one of the two outer portions of the gear can be rotated on the splines to a position that will allow proper clearance between the meshing screws to limit the internal leakage (slip) in the pump. These gears time up in at least six places per revolution.
New Timing Process
Where the timing process for twin screw pumps normally took 16 man hours (minimum), it now takes approximately three to five minutes. Two-piece timing gearssimplify the timing process to the following steps:
Place the hubs on the shafts.
Leave the driven shaft stationary, then turn the drive shaft as far as possible in one direction using a pointer and mark that splined tooth.
Turn the drive shaft in the opposite direction as far as possible and mark that splined tooth.
Count the number of splines between the two marks and divide by two, then turn the shaft to that position. This gives you the proper axial alignment of the inner-meshing screws.
Mesh the two outer portions of the gears together and slide them onto the hubs.
The timing of the pump is complete. The seals and bearing can be changed and the timing done in place without having to go to the machine shop.
A chemical company in Orange, TX, asked their pump rebuilders to install two-piece timing gears into their pumps about five years ago. No machine work or factory overhaul has been done on the pumps; they just change the bearings and seals when necessary. The gears have not shown wear.
In a different company, a pipeline pump that runs at 3600-rpm normally breaks down in about six months. The bearings fail and lock up, the main driver breaks the shaft and both timing gears shear their keyways and spin on the shaft. Two-piece timing gears on the same pump, in contrast, are less likely to damage and are useable for a second run. Usually the gears gauld up on the shaft making the gears unusable, but two piece timing gears allowed the company to reuse the driven shaft because the gears didn't mark the shaft. They only had to make the driving shaft to rebuild the pump.
One company reports a tripling (or more) of pump life through "careful timing gear maintenance."2 Consider it as one way to help extend the life of your pumps.
1. U.S. Patent Numbers # 6,093,009 and #6,301,782 B1
2. Sier-Bath Twin Screw Pumps Instruction Manual (Manual # 2166-EI8606), Worthington Dresser, Worthington Pump division, Bradford, Ontario, Canada, page 4 and 23.