LOAD DISTRIBUTION AND TOOTH CONTACT INSPECTION

 

Blog: https://componentesmecanicos.blogspot.com/                    By: Eduardo Niño de Rivera

                                                                                                       Edition: John Amendola Sr.                                                                                                                                         Artec-Machine Systems.

 

INTRODUCTION.

This article is intended for general mechanical maintenance personnel with a higher level of training than what is normally required as described in the previous article, “Gearbox Inspection for Non-Experts”. Properly checking and recording the contact pattern between gear teeth requires specific knowledge to allow the maintenance personnel to follow the condition of the gear and any degradation that may occur over time. This precise and complete information can be forwarded to gear specialists who can then prepare an analysis, diagnosis, potential problems and make recommendations for corrective action.

 

Gear capacity calculations assume that during operation the load will be evenly distributed throughout the entire tooth contact surface. If this condition is not met, contact stresses will be higher in specific localized areas of the tooth flank thereby reducing gear life. This article discusses the reasons and consequences of an inadequate load distribution and the correct way to conduct an inspection of the tooth contact.

 

1. PROPER CONTACT

Contact stress for an evenly distributed load on a surface area is given by the load divided by the surface area. The most common units are, kg/cm², Mpa or lb/in² (PSI).

 

Gear manufacturers try to achieve an even load distribution over the entire tooth contact area under intended working conditions, otherwise, contact stresses will increase in specific areas, leading to premature gear failure.

 

         

This is an example of uneven load distribution. Contact stress increases from left to right.

 

2. CAUSES FOR AN UNEVEN LOAD DISTRIBUTION

Several factors can contribute to a poor load distribution.

 

2.1.- SHAFT MISALIGNMENT

Misaligned shafts create an uneven tooth contact with poor load distribution. The contact pattern will depend on the plane of misalignment relative to the load, e.g., if shaft misalignment is in the same plane as the load, the stresses will be greater on one side of the tooth.

 

Photo courtesy GEARTECH © 2003.

 

 

In this case, misalignment caused greater contact stresses toward the left end of the gear, leaving the right side almost untouched.

 

But if misalignment is in a plane perpendicular to the load, tooth contact will be displaced toward the root at one end and toward the tip at the other end.

 

2.2.- GEAR DEFORMATION DURING OPERATION.

There are several reasons why gears undergo deformation.

1.       Rotor (shaft and gear) elasticity in:

a.       Bending, and,

b.       Torsion

2.       Temperature differences

a.       Within the gear

b.       Along the gear tooth.

3.       Casing distortion due to improper installation

 

2.2.1.- Rotor elasticity allows two types of deformation, the first one is due to the bending moment caused by the load acting on the rotor supported by two journal or radial bearings.

 

The second is due to torsion from the transmitted torque.

The shaded area on the top drawing shows the contact pattern in an unmodified tooth with no load. The bottom drawing shows the contact pattern under torsion due to a torque load.

 

2.2.2 THERMAL DEFORMATION

Helical gears also have a pumping effect, forcing the lubricant to flow along the tooth, creating a temperature difference within the tooth and an uneven thermal expansion along the tooth flank.

During operation, heat is generated by friction from the churning of the oil and air mixture dynamically compressed in the gear mesh, and the shearing of the oil film in the bearing elements and seals. Heat is also generated by the windage created by the rotational speed of the gear rotors. In gear units where there is dipping of the gear rotor in the lubricant, heat will develop due to the churning of these components inside the oil sump. At the same time, heat is dissipated through the casing walls and transferred by the circulating oil away from the points where it is generated. The end result is an uneven temperature distribution within the gearbox.

 

2.2.3 CASING DISTORTION DUE TO IMPROPER INSTALLATION

Geometrically, three points define a plane, if a fourth point is introduced, it will probably fall outside of this plane. We can see this in a table with four legs, where one leg must be wedged to touch the ground simultaneously with the other three. The same thing can happen in a gearbox, a condition called soft foot. If during installation all the nuts are tightened without properly shimming the base, the casing will distort, and the resulting misalignment in the rotors may have a negative impact on load distribution in the gear teeth.

 

To avoid casing distortion, before tightening the nuts, the base must be shimmed at the anchor points where there is a gap.

We will discuss this subject in more depth in a future article.

 

2.2.4 TOOTH PROFILE MODIFICATION

Manufacturers may modify the tooth profile to optimize the operating slide roll contact ratio in the gear mesh that undergo considerable deformation due heavy loads or high speed. Although the static no-load contact in these gears may be uneven, the dynamic contact under a specific load and speed will be correct.

3. OTHER TOOTH PROFILE MODIFICATIONS.

Gear teeth are relatively rigid, magnifying the impact of minor manufacturing defects. Other tooth profile modifications can be made to the tooth profile to compensate for these defects. 

The tip and root of the tooth may be relieved to minimize the impact at the points of tooth contact and release.

 

Tooth profiles may also be modified to compensate for deformations due to:

 

1.       Tooth bending as a loaded cantilevered beam

2.       Shear stresses on loaded teeth

3.       Stress concentration due to manufacturing defects  

4.       Temperature difference between gear teeth and hub

5.       Shock loads induced by the motor or the driven machine

6.       Centrifugal force on gears rotating at very high speed

 

 

4. NOMINAL GEARBOX RATING

Gear capacity calculations assume an even load distribution over the entire tooth contact surface. As discussed above, gear quality plays an important role in meeting this assumption, so does operating the gearbox under the load and speed considered in the design, but plant maintenance and production personnel also play an important role on gearbox life (“Gearbox Inspection por Non-Experts”).

5. GEAR TOOTH CONTACT INSPECTION

When plant personnel report unusual noise, vibration or temperature in a gearbox or when a gearbox is reassembled, two types of contact inspections must be made:

1.       Static, without movement and under minimal load; and,

2.       Dynamic, preferably at operating load and speed.

 

 

5.1. Static inspection may be done on an open gearbox, but bearings must be fixed in place.

 

5.1.1 PROCEDURE

a.       Clean gear teeth with quick drying solvent, teeth must be oil free.

b.       Apply a very thin layer of hi-spot checking fluid, such as Dykem Prussian Blue, on both sides of at least three teeth of the low-speed gear (the larger one).

c.       Holding one rotor by hand, turn the other one, also by hand, until the checking fluid is transferred to the high-speed gear, then reverse the direction of rotation to transfer the checking fluid to the other face of the high-speed gear teeth.

 

 

                                                                        If too much ink is applied, it will be transferred to the other gear showing full contact whether it is true or not.

 

For gears with modified tooth profile, although tooth contact may be correct during operation, static inspection will show an uneven contact. Some manufacturers provide the correct static contact pattern for their gears. If this information is not available, the contact pattern in the non-loaded tooth flanks will be an indicator of gear alignment. However, loaded face contact inspection patterns must be recorded to keep track of their evolution over time.

 

Use a transparent adhesive tape to record contact pattern. The tape must cover the entire tooth flank.

 

1.       Apply the tape on the tooth flanks where the bluing has been transferred.

2.       Make sure the tape is completely clean, no fingerprints or debris.

3.       Rub the entire surface with a clean cloth to ensure full and smooth contact.

4.       Mark the tape to identify the gear tooth root and tip.

5.       Place a clean white paper about 6 inches longer than the tooth length on a flat surface.

6.       Starting at one end, peal the tape off in a sharp angle.

7.       Apply the tape on the clean paper.

 Identify

a.       Loaded and unloaded flank.

b.       Tooth tip and root.

c.       Coupling and free end.

 

 

5.2. Dynamic inspection is done with quick drying layout fluid. If possible, the test should be performed with the gearbox properly lubricated and at operating speed and load.

 

a.       Clean gear teeth with quick drying solvent, teeth must be oil free.

b.       Apply a very thin layer of Dykem layout fluid on both flanks of at least three groups of three teeth each, on both, the high and low speed gears.

c.       Allow the layout fluid to dry (about two minutes).

 

The Dynamic contact pattern is given by the area where the layout fluid has worn out.

 

6.- EVOLUTION OF WARE

Contact inspection is necessary during reassembly and alignment to ensure proper gearbox operation when it is recommissioned. It is also necessary to compare recorded data over time to follow the evolution of gear and bearing deterioration. An accelerated tendency in ware patterns provides advanced warning to replace parts and make timely adjustments to keep machines producing within specification and to avoid costly shutdowns due to unexpected gearbox failures.

 

7.- CONCLUSION

Misalignment and tooth deformation prevent proper tooth contact and may shorten gear life. When plant personnel report unusual gearbox noise, vibrations or temperature; when parts are replaced; or when adjustments are made to a gearbox, tooth contact must be checked in two ways: static, with high-spot checking fluid, turning the gears by hand to see the pattern of fluid transferred from one gear to the other; and dynamic with quick drying layout fluid to observe the ware pattern under normal operating conditions. These inspections are necessary at reassembly and alignment to ensure proper gearbox operation. Comparing the recorded data over time, shows the evolution of gear and bearing ware, providing advanced warning to replace parts and make timely adjustments to keep production within specification and avoid the high costs of unexpected shutdowns.

 

ACKNOWLEDGEMENT:

This article heavily relies on Part 1, Load Distribution, Artec-Machine Systems “Gearbox Field Inspections” seminar, AGMA Gear Expo – 25 OCT 2017,

 

REFERENES 

 

1.- https://www.horsburgh-scott.com/resources/PDFs/hs-maint-manual.pdf

 

2.- https://www.powertransmission.com/articles/0314/Best_Practices_for_Gearbox_Assembly_and_Disassembly/

 

3.- https://www.machinerylubrication.com/Read/28765/how-to-inspect-a-gearbox-

 

4.- https://www.engineerlive.com/content/top-10-tips-industrial-gearbox-inspection-and-maintenance

 

5.- https://fieldservicesengineering.co.za/gearbox-maintenance/

 

6.- .- https://webstore.ansi.org/SDO/AGMA?gclid=Cj0KCQiA7oyNBhDiARIsADtGRZYWHCXT9-PAX-wSTWUucvcovdbsBX5FYce-NdGmpqgtYP6F96ecO4waAsavEALw_wcB ASI/AGMA 1010-F14 Appearance of Gear Teeth – Terminology of Ware and Failure. Febrero 2020

 

7.-https://www.geartechnology.com/issues/1192x/faure.pdf Classification of Type of Gear Tooth Wear – Prat I

 

8.- https://www.geartechnology.com/issues/0193x/faure.pdf Classification of Type of Gear Tooth Wear – Prat II

 

9.- The Speed Reducer Book Peerless-Winsmith Inc. 1980  

 

10.-The Battle for Reliability, Joel Levitt, 2021, Springfield Resources

 

11.- The Quest for Defect Elimination, Joel Levitt, 2020, Springfield Resources