The Evolution of Gear Spindles - Emerson Industrial Automation

Proceedings of DETC2000 

2000 ASME Design Engineering Technical Conference 

September 10-13, 2000, Baltimore, Maryland, USA 

DETC2000/PTG14454 

THE EVOLUTION OF GEAR SPINDLES 

Jon R. Mancuso 

Engineering Manager 

Kop-Flex, Emerson Power Transmission Corp. 

Joe Zilberman 

Manager Research 

Kop-Flex, Emerson Power Transmission Corp. 

ABSTRACT 

Gear spindles were first applied to rolling mills in the early 

1950’s. Over this time the basic design has remained constant 

except for the material used and the hardening process. And 

today they are still the most commonly used drive for rolling 

mills. This is because they are power dense (handle the highest 

torque for a given Outside Diameter), easy to maintain and 

economical. New processes have been developed for grinding 

carburized gear spindles to optimize tooth contact thereby 

increasing spindle capacity and life by 30-100%. This process 

allows for a more compact design (usually one size smaller) 

along with improved life. 

INTRODUCTION 

Prior to the 50s wobblers and slipper spindles (see Figure 1 & 

2) were primarily used in steel mills. During the 50’s gear 

spindles were first applied to hot rolling and cold mills. Over 

the years the basic design has remained constant, but there have 

been many features incorporated into the design and several 

changes in how they are optimized, analyzed, and made. 

Figure 2. 1950 VINTAGE MILL USING SLIPPER 

SPINDLES 

Original designs employed carbon steels which were induction 

hardened and were very basic in their design 

Figure 1. Typical wobbler, and slipper spindles 

1950 Vintage Gear Spindle (See Figure 3) 

Features 

1. One piece roll sleeves 

2. Induction hardened or nitrided 

3. Thrust button off center 

4. Hubs keyed to shaft 

5. Lip seals 

1 Copyright © 2000 by ASME

Figure 3. 1950 Vintage Gear Spindle 

Today’s gears spindles incorporate many features to 

accommodate the higher power, smaller diameters, easily 

maintainable improved life, higher misalignment with higher 

reliability. 

. 

2000 Vintage Gear Spindle (see Figure 4 & 5) 

Features 

1. Usually replaceable gear element unless OD 

restricted – for ease of replacement and at 

economical cost. 

2. Nitrided or corrected carburized gear element – 

for longer life. 

3. Thrust Button on center – for optimum wear life. 

4. Hubs splined to shaft - to maximize torque. 

5. Rising rings seals – to keep lube in and rolling 

solutions out. 

6. Piloted roll end pods – for lower wear. 

Figure 5. 2000 VINTAGE HOT STRIP MILL USING 

GEAR SPINDLES 

Nitriding vs Carburizing 

Today most gear spindles are either nitrided or carburized. For 

reduced distortion, Nitriding is used. Nitriding provides a hard 

case which range from Rc 55/64 and with very little distortion. 

The case depth ranges from 0.015” to 0.030” (0.38-0.76mm). 

This process is good for fine pitch gearing in bar, rod and cold 

mills. For roughing mill and hot strip mill spindles with course 

pitch teeth a deeper case is required. These spindles employ 

carburized gearing which produces deep cases 0.060” to 0.250” 

(1.5-6.4mm) and a hardness of Rc 55/62. Again like induction 

hardening during the quench operation, distortion occurs to the 

actual tooth and also pitch diameter. 

During operation, gear spindles are subject to high 

misalignment. At 2 degrees only 40% of the teeth carry the 

load. The limited number of teeth carrying the load combined 

with the distortion, resulting from carburizing, causes some 

teeth to be highly loaded. This results in subsurface shear and 

spalling. The results of this distortion (if uncorrected) shows up 

as areas of spalling. 

Figure 4. 2000 Vintage Gear Spindle 


How A Gear Spindle Works 

Most gear spindles have involute profiles teeth with the external 

teeth crowned. One must understand how a gear coupling 

works in-order to understand why distortion, material, 

lubrication, misalignment and lubrication are critical for the 

successful operation of spindles. When a gear coupling is 

misaligned (pivot position) with no load, only two teeth 90 

degrees from the plane of misalignment (tilt position) come into 

contact (see Figure 6). As a tooth is rotated 360 degrees under 

load, its point of contact will then move along the tooth in a 

hour glass pattern (see figure 7). Depending on the load, teeth 

may not contact at the center (pivot point) and heavier pattern at 

the end of the pattern (the tilt position). The width of the pattern 

depends on what the curvature it was cut for and what the 

operating misalignment is. If the gear teeth are cut for 4 

degrees and it runs at 2 degrees one will see a sweep pattern 

approximately 50% of the length of the tooth. 

Figure 7. TOOTH LOAD SWEEP 

(THROUGH 360 0 ) 

VARIOUS TOOTH FORMS (See Figure 8) 

Spindle teeth are usually of two forms 

1. Tip piloted 

2. Root piloted 

The profiles are usually cut for 20 or 25 degree pressure angles. 

FIGURE 8. VARIOUS TOOTH FORMS 

Figure 6. LOADING CYCLE OF GEAR COUPLING 

TEETH 

Tip piloted teeth are more common in nitrided spindles. 25 

degree pressure angle teeth are commonly used where teeth cut 

for high angles (4-6 degrees) the ends of the teeth can get pretty 

thin and during roll changing or even during operation the teeth 

can break in bending. Twenty (20) degree pressure angle teeth 

have a slightly sharper curvature along their profile and make a 

sharper parabolic ball which effectively improves the rolling 

action and can slightly derease the coefficient of friction; this 

tends to make 20 degree teeth more suited where wear is of 

concern (high ScV). Unfortunately the right tooth design is not 

a precise science and which is best for which application has 

developed for experience. One form failed and another tried in 

the same application and; “success”. 


MATERIALS 

There are various materials used in gear spindles. The 

physical properties of a gear spindle depends on the material, 

the heat treatment, and the process used to shape teeth after heat 

treatment. Many alloys are used to make spindles, each with its 

own inherent core strength and other properties. Manufactures 

then usually harden the teeth to increase wear resistance and 

strength, using either induction hardening, nitriding, or 

carburizing. The effects of each type of heat treatment differ 

for each alloy. One material and heat treatment is not best for 

all applications. There are three things that one must evaluate, 

Strength (see Table 1), Wear (see Table 2), and ScV (see Table 

4). The tables below compare these properties for various 

materials, heat treatment and also the type of lubrication 

Strength Hierarchy of Gear Coupling Materials 

Material Minimum 

Hardness 

Relative 

Strength 

(BHN) 

3310 Carburized 360 5 

4320 Carburized 310 4 

Nitralloy N 400 3.5 

8620 Carburized 300 3.5 

4140 Fully Contoured 300 3.25 

Induction Hardened 

Nitralloy 135 320 2.75 

4340 Nitrided 330 2.75 

4140 Flank Induction 270 2.5 

Hardened 

4140 HT and 310 2.5 

Nitrided 

1035/1045 Fully 155 1.75 

Contoured Induction 

Hardened 

1035/1045 Induction 155 1 

Hardened 

1035/1045 155 1 

Table 1. Strength Hierarchy 

Wear Hierarchy of Gear Coupling Materials 

Material Minimum 

Hardness 

Relative 

Strength 

(BHN) 

Nitralloy N 65 Rc 4.5 

Nitralloy 135 65 Rc 4.5 

3310 Carburized 62 Rc 3 

4320 Carburized 60 Rc 2.5 

8620 Carburized 58 Rc 2.25 

4140 HT and 54 Rc 2 

Nitrided 

4140 Induction 50 Rc 1.75 

Hardened ALL 

4340 Nitrided 49 Rc 1.5 

1035/1045 Fully ALL 45 Rc 1.5 

4140 Heat Treated 280-310 1.25 

BHN 

1035/1045 ALL 155 BHN 1 

Table 2. Wear Hierarchy 

ScV Limits for Materials and Lubrication 

This factor looks at the contact (hertzian) stress and the 

sliding velocity of the teeth from misalignment. It also 

considers the type of lubrication. 

Most gear spindles today are lubricated with special 

greases that were specially developed for spindle operation. 

There are two basic types of lubricants. The lithium based 

grease with base oil viscosity over 150 SUS @ 210 o F, also has 

several additives, for water washout, handling high impact loads 

(molydifsulfide 5-10%), etc. The other types of lubricants used 

are special synthetic greases that can withstand high ScV’s seen 

in the latter stands of Hot Strip Mills and many Cold Mills. 

These mills may operate over 500 RPM and operate at angles 

over 1 degree. They will have ScV’s over 500,000 psi-ips. 

There are two parts of this ScV factor 

1. Compressive stress (hertz stress) and tooth sliding 

velocity 

Hertz Compressive Stress (assumes a cylinder on a 

plane) 

S c = 4580 

x 

T 

ρ x N x PD x D c x h 

Where 

T = Torque 

ρ = Number 

N = Number of teeth 

PD = Pitch Diameter 

( lb -in) 

of teeth in contact 

( in) 

(%) 

h = Tooth engagement height (1.6/DP 

) 

(in) 


One of the most important factors in the design of gearing 

for gear couplings/spindles is the number of teeth engaged 

under load. This varies not only with the sleeve and hub 

design but also with misalignment, load, heat treatment and 

manufacturing process. As a rule of thumb, generally the 

number of teeth loaded is primarily affected by 

misalignment (see Table 3). The numbers are further 

reduced because of distortions from heat treating and other 

manufacturing errors. Carburizing distortions can reduce 

tooth loading, so only 25-50% of the number in the chart is 

achieved. Ground vs hob/shaped teeth may increase the 

number of teeth in contact by 30-50%. The design of the 

sleeve and hub can increase or decrease these numbers. 

Sleeves and hubs with thick rims may not allow the teeth to 

deflect under load or if to thin cause all teeth to be loaded. 

Having all the teeth loaded sounds good but for spindles 

running at high ScV having them go through a no load cycle 

allows the lube to squeeze through and this could allow the 

coupling to run cooler and may keep the lube from breaking 

down. 

Misalignment Degrees Percent of teeth in contact 

0 80% 

0.5 60% 

1 50% 

2 40% 

3 35% 

4 25% 

5 15% 

6 10% 

Table 3. Percent of Teeth Loaded vs Misalignment 

2. ScV Criteria 

S c V a product of Hertz stress above and the sliding 

velocity of the gear teeth 

V = π 

Where 

PD = Pitch Diameter 

α 

⎡ RPM ⎤ 

x PD x sinα x ⎢ ⎥ 

⎢⎣ 

60 ⎥⎦ 

= Operating angle 

(in) 

(degrees) 

(in/sec) 

S c = Hertz Stress @ operating angle 

Limits for ScV are based on the type of material, surface 

treatment and the type of lubrication (see Table 4) 

Material Lubrication ScV (psi-ips) 

Method 

Medium Carbon Grease 125,000-175,000 

Steel AISI 1032-1045 

Alloy Steel (280-310 Continuously 175,000-225,000 

BHN) 

Lubricated 

Alloy Steel (280-310 Grease 200,000-300,000 

BHN) 

AISI 4140 (nitrided) Continuously 200,000-300,000 

Lubricated 

AISI 4140 (nitrided) Grease 400,000-600,000 

AISI 8620/4320 Grease 300,000-400,000 

(Carburized) 

AISI 3310 

Grease 400,000-500,000 

(Carburized) 

Nitralloy 135 & N 

(Nitrided) 

Grease 1,000,000-1,200,000 

Table 4. ScV Criteria 

NITRIDED SPINDLES. 

Nitrided gear spindles are commonly used in bar mills, cold 

mills and some finishing stand in hot strip mills. This surface 

finish provides good wear resistance. The harder a surface is, 

the more it resists wear. 500 BHN (52 Rc) is approximately 

three times more wear resistant than 200 BHN, but 600 BHN is 

approximately twice as resistant as 500 BHN, 

CARBURIZED SPINDLES 

One method of correcting this distortion is lapping; the 

“rubbing” of the external tooth with the internal tooth using an 

abrasive medium to “wear the parts in” or remove the high 

spots. The difficulty here is that the parts are lapped in matched 

sets, If they are mismatched the tooth contact will be reduced 

and may be greater than if it was not lapped. One approach to 

minimize poor contact from a possible mismatch is to index lap 

the parts. This improves the contact if mismatched but does not 

produce the best contact.. 

Several years ago, the existing tooth forms and limitation of 

grinding equipment prevented gear spindle teeth from being 

ground after carburizing. With the advent of the compact hot 

strip mill came the need for higher angles and power with 

smaller spindles. A patented tooth form and process for grinding 

carburized gear spindles to optimize tooth contact was 

developed to meet these new requirements. This design and 

process corrects carburized tooth distortion in the internal and 

external gear tooth flanks. This gearing has been in service in 

one application for over three years where the teeth still have a 

mirror finish. It also has been used in other application with 

similar results. 


This gearing provides the following features 

1. Ground to AGMA 10-11 for improved wear life and 

reduced tooth spalling 

2. A unique process and tooth design that reduces tensile 

stress due to grinding 

3. Grinding increases the number of teeth in contact, resulting 

in longer operating life 

4. More teeth in contact equals greater torque capacity, 

smaller spindles and larger service factors 

Below are pictures of carburized spindle 

Figure 9. Uncorrected 

Figure 10. Lapped 

Figure 11. Ground 

BLUE CONTACT PATTERN 

AFTER 3 YR OF OPERATION 

Figure 11. CARBURIZED AND GROUND GEARING 

(CGG) 

OTHER ADVANCES IN THE TECHNOLGY USED 

IN GEAR SPINDLES 

Today FEA analysis is used in the design of critical components 

for gear spindles. 

BLUE PATTERN 

AFTER ONE YR OF OPERATION 

FIGURE 9. AS CARBURIZED GEARING 

In one application the customer wanted the pinion sleeves (see 

Figure 12 & 13) to have a very heavy shrink 0.003/in of 

diameter. These spindles had a replaceable gear ring (see 

Figure 14). It is important to assure that these gear rings are 

properly supported or the intermediate sleeve can rock during 

operation and cause excessive wear of the teeth or splines. A 

FEA analysis and 1/8 scale model were done and it was 

determined that the pilots, front and back of the intermediate 

sleeve had to be designed differently. The front pilot grew 

(0.048”) but the back pilot actually collapsed (0.009”). This 

would have made assembly very difficult. The picture above of 

the CGG after 3 years demonstrates that this gear is properly 

loaded. 

BLUE CONTACT PATTERN 

AFTER 1 YR OF OPERATION 

FIGURE 10. CARBURIZED AND LAPPED 

GEARING 


FEA models can also be used to better predict the load patterns 

of the gear teeth (see Figure 15A,B, C) . This analysis is very 

complicated and is used to optimize tooth shapes. Further work 

in analyzing tooth loading is on going Since there are many 

variables in the design of gear spindles one still must rely on 

conventential analysis and experience factors to design, 

optimize and predict life. 

Figure 12. FEA Model of Pinion Sleeve 

A. 

Figure 13. Pinion Sleeve 

B. 

C. 

FIGURE 15. TOOTH LOADING OF GEARING 

Figure 14. Gear spindle pinion end with intermediate 

Sleeve 


FEA is also used to evaluate failures and improve designs. 

Normally when a cobble in a mill occurs the roll neck will snap 

or the shaft in the spindle will shear? One customer was 

experiencing repetitive failure of roll sleeves (see Figure 16). 

Examination of the failure indicated that failure was initiated 

near a lube fitting in this sleeve. A model (see Figure 17) was 

run and the failure mode and path of the crack was predicted 

(see Figure 18 A & B). Further analysis was done to reposition 

the lube fitting further back in the roll sleeve. This increased 

the safety margin be 50%. Eventually the lube fitting was 

eliminated in the roll sleeve totally. Lubrication fittings were 

put into the shaft. This not only eliminated the roll failures but 

also positioned the fitting so it was more accessible thereby 

assuring it would get lubricated. 

Figure 18 A & B. Failure Mode Prediction 

Figure 16. Roll Sleeve 

Conclusions 

Gear spindles continue to be the most common spindle 

used on main drive mills. They are very power dense and if 

properly maintained they can provide in-excess of three years of 

life before there gear elements need to be replaced. It is also 

expected that further improvements in analysis, materials and 

process will allow the gear spindle to be the primary drive 

coupling in steel mills for years to come. 

REFERENCES 

Mancuso, Jon “Couplings and Joints, Design, Selection, and 

Application” Marcel Dekker, 2 edition 1999. 

Kop-Flex Mill Product Catalogue KMP-99 

US Patent – 6,026,700 Tooth Form Parameters for Ground 

teeth of Gear Spindle Coupling and Method of Making the 

Same 

MIL-C-23233, Coupling for Propulsion Units, Auxiliary 

Turbines and Line Shafts Naval Shipboard 

Figure 17. FEA Model of Roll Sleeve 

.

The Evolution of Gear Spindles - Emerson Industrial Automation

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