What are Gear Boxes? Types | Working Principle | Technical Specs for Special Applications

This article is about What is Gear Box?, Gear Boxes Types, Functions, Working Principle and how it is used for special purpose applications in plants and refineries. More will be discussed about Technical Specification of Gear Boxes / Units Used in Plants and Refineries for engineers, design managers and supervisors of Rotating Equipment Engineering.

1. SCOPE

2. GENERAL

3. ADDENDA The following section, subsection numbers refer to API 613, Fourth Edition

Section 1 – General

1.2 Alternative Designs

1.3 Conflicting Requirements

1.4 Definition of Terms

1.5 Referenced Publications

Section 2 – Basic Design

2.1 General

2.2 Rating

2.3 Casings

2.4 Casing Connections

2.5 Gear Elements

2.6 Dynamics

2.7 Bearings

2.8 Lubrication

2.9 Materials

2.10 Nameplates and Rotation Arrows

Section 3 – Accessories

3.2 Couplings and Guards

3.3 Mounting Plates

3.4 Controls and Instrumentation

3.5 Piping and Appurtenances

Section 4 – Inspection, Testing and Preparation for Shipment

4.2 Inspection

4.3 Testing

4.4 Preparation for Shipment

Section 5 Vendor’s Data

5.1 General
Appendix H – Gear Tooth Inspection

Figure H-1 Tooth Alignment (Lead) Modification of Helical.
Pinion Figure H-2 Profile Modification.

Table I – Driver Trip Speeds

Table – II Minimum Gear Service Factors

What are Gear Boxes or Gear Units?

A gearbox, also known as a gear box or transmission, is a mechanical device that is used to control and transfer power from a power source, such as an engine or motor, to the driven machinery or equipment. It consists of a system of gears that are arranged in a specific configuration to achieve the desired speed, torque, and direction of rotation.

The main purpose of a gearbox is to convert the rotational speed and torque generated by the power source into a form that is suitable for the driven machinery. This allows for the efficient transfer of power and the adaptation of speed and torque to match the requirements of the application.

Gear Boxes are commonly used in various industries and applications, such as automotive vehicles, industrial machinery, agricultural equipment, construction machinery, and power generation systems. They are essential components in systems where the input speed and torque need to be adjusted or optimized for the output requirements.

Gearboxes can have different types of gears, such as spur gears, helical gears, bevel gears, worm gears, or planetary gears, depending on the specific application and performance requirements. They are typically housed in a protective casing and may include additional features like lubrication systems, cooling mechanisms, and control mechanisms for gear shifting in automotive applications.

Overall, Gear Boxes play a crucial role in power transmission, allowing for the efficient and controlled transfer of power between different components of a system.

types of gear boxes

There are several types of Gear Boxes, each designed for specific applications and operating conditions. Here are some commonly used types of gearboxes:

1. Spur Gearbox: Spur gears are the simplest and most common type of gears. A spur gearbox consists of cylindrical gears with straight teeth that mesh together. They provide high efficiency but produce noise and vibration.
2. Helical Gearbox: Helical gears have angled teeth that are cut at an angle to the gear axis. Helical gearboxes offer smoother and quieter operation compared to spur gears and can handle higher loads. They are commonly used in automotive and industrial applications.
3. Bevel Gearbox: Bevel gears are cone-shaped gears that transmit power between intersecting shafts. Bevel gearboxes are used when the direction of rotation needs to be changed by 90 degrees. They are commonly found in applications such as differential drives and power tools.
4. Worm Gearbox: A worm gearbox consists of a worm gear (a screw-like gear) and a worm wheel. It provides high reduction ratios and is used in applications where high torque and low speed are required, such as conveyor systems and lifting equipment.
5. Planetary Gearbox: Planetary gearboxes, also known as epicyclic gearboxes, consist of multiple gears arranged in a planetary configuration. They offer high torque density and compact size. Planetary gearboxes are commonly used in automotive transmissions and industrial machinery.
6. Cycloidal Gearbox: Cycloidal gearboxes use cycloidal motion to transmit power. They consist of eccentric cams and roller pins that create a smooth and compact gear mechanism. Cycloidal gearboxes are known for their high torque capacity and durability.
7. Parallel Shaft Gearbox: This type of gearbox is commonly used in gas turbines and steam turbines. It consists of two or more parallel shafts with gears of varying sizes. It provides a compact and efficient solution for transmitting power from the turbine to the generator or other driven equipment.
8. Integral Gearbox: In some turbine designs, the gearbox is integrated into the turbine housing or casing. This helps in reducing the overall size and weight of the turbine system. Integral gearboxes are commonly used in small-scale turbines or certain types of compact turbines.

These are just a few examples of Gear Boxes, and there are many other specialized types available for specific applications. The selection of the gearbox depends on factors such as the desired speed, torque, efficiency, noise level, and the specific requirements of the application.

It’s important to note that the specific type of gearbox used in a turbine depends on various factors, including the type of turbine (e.g., gas turbine, steam turbine, wind turbine, hydro turbine), power output requirements, rotational speed, torque, and other design considerations.

Function of Gear Box

A gearbox has the primary function of transmitting and controlling mechanical power from a power source to an output device. It allows for the conversion of rotational speed and the amplification or reduction of torque.

Additionally, gearboxes enable directional control by changing the rotation direction and facilitate load distribution among multiple gears. Ultimately, gearboxes play a crucial role in efficiently transmitting power and ensuring the desired performance in various mechanical systems.

Gear Box Working Principle

The working principle of a gear box is based on the interaction between gears to transmit and control mechanical power. The gear box contains multiple gears of different sizes and configurations that are arranged in a specific sequence.

When the input shaft rotates, it drives the first gear, which then engages with and drives the subsequent gears. The gears mesh together and transfer rotational motion from the input shaft to the output shaft.

The gear ratios determine the speed and torque conversion within the gear box, allowing for speed reduction or amplification depending on the gear arrangement. The smooth operation of the gear box relies on proper lubrication and precise alignment of the gears.

Technical Specification of Gear Boxes / Units Used in Plants and Refineries

1. Scope

American Petroleum Institute (API) standard 613, Fourth Edition, June 1995 is adopted as a SABIC Engineering Standard (SES) with the following addendum.

2. General

2.1 Special purpose gear Boxes manufactured and packaged to this standard shall comply with API 613, Fourth Edition, except as modified herein or as set forth in SES G07-X02, project specifications, or the purchase order.
2.2 Special purpose Gear Boxes shall be supplied by Vendors qualified by experience to manufacture and package the units being proposed. Vendor shall be considered qualified if, at the location proposed for manufacture and packaging, they have previously manufactured and packaged two gear units of the same model as that being proposed, which meet the following requirements:

• Gear units being proposed shall be of identical design to the gear units on which experience is based.
• The type of gear (double helical, single helical), materials of the gear elements, and gear to shaft attachment method shall be identical to the gear units on which experience is based
• The gear tooth hardening and finishing methods and resulting hardness and surface finish for the gear units proposed shall be identical to the gear units on which experience is based.
• The gear tooth contact stress and bending stress of the gear units proposed shall be equal to or less than the gear units on which experience is based
• Gear rated power and pitch line velocity for the gear units proposed shall be equal to or less than the gear units on which experience is based
• The speed range of the gear units proposed, shall not exceed that of the gear units on which experience is based.
• Protective and auxiliary systems of the gear units proposed shall be similar in type and design to the gear units on which experience is based.
• The types and sizes of bearings of the Gear Boxes proposed shall be identical to the gear units on which experience is based.
• The gear and bearing lubricant and methods for application of the lubricant on the gear units proposed shall be identical to that of the gear units on which experience is based.

2.3 The two identical gear units shall have been in continuous operation for a minimum of 8,000 hours each, and shall have performed satisfactorily throughout their operation without major damage to the gear elements or bearings.
2.4 Proof of compliance with the above requirements shall be submitted to SABIC with the equipment proposal for approval by SABIC.

2.5 Where vendor qualification requirements above prevents the application of the latest technology, vendor may submit a second or alternative proposal incorporating the latest technology features for evaluation by the Owner’s Engineer. This alternative proposal shall specifically identify the features undemonstrated by experience, and state their advantages.

3. Addenda

The following section, subsection, paragraph numbers refer to API 613, Fourth Edition.
Section 1 – General
1.2 Alternative Designs
Substitute the requirement as follows:
Vendor may offer alternative designs. Vendor’s primary quotation shall be in accordance with this standard.
Any exceptions, including design differences, shall be clearly stated in the proposal, as required by 5.2.1.
Vendor shall not change the component suppliers identified in the quotation or during bid evaluation without SABIC written approval.
1.3 Conflicting Requirements

Add the following sentence:
In case of conflicting requirements in the request for quotation, purchaser order, or specifying documents, vendor shall seek guidance from SABIC before proceeding.
1.4 Definition of Terms
Add the following new paragraphs:
1.4.23 Scuffing: Is a form of gear tooth surface damage which refers to welding and tearing of the tooth surface by the flank of the mating gear. Scuffing occurs when oil film thickness is small enough to allow the flanks of the gear teeth to contact and slide against each other.
1.4.24 Lead Modification: Is a calculated machined deviation of the pinion flank from the theoretical tooth
form, intended to improve uniformity of the load distribution across the face width of the mating gears.
1.4.25 Special tool: A tool that is not a commercially available catalog item.
1.5 Reference Publications

Add the following paragraphs:
SABIC and vendor shall mutually determine the measures that shall be taken to comply with any
governmental codes, regulations, ordinances or rules that are applicable to the equipment.
Some of the API 613, Fourth Edition Industry Standards referenced have been updated since publication.

The latest edition of the referenced standard in existence at time of issue of the Request for Quotation shall apply unless the update changes the original intent of the reference. The following standards are part of this addendum. The latest issues, amendments and supplements to these documents shall apply unless otherwise indicated.

SABIC Engineering Standards (SES)

E02-E02 Hazardous Area Classification
E19-S01 Packaged Equipment Electrical Requirements
G03-S01 Lubrication, Shaft-Sealing, Control Oil Systems and Auxiliaries
G12-S01 Special Purpose Couplings
G19-S01 Machinery Protection Systems
S20-G01 Plant Equipment Noise Levels
X01-S03 Instrumentation for Packaged Equipments

International Standards Organization (ISO)

1328-1 Cylindrical Gears – ISO System of Accuracy.
6336-5 Calculation of Load Capacity of Spur and Helical Gears.
TR 10064-1 Cylindrical Gears – Code of Inspection Practice.

2. Gear Box Basic Design

2.1 General
2.1.4 Substitute the requirement as follows:
Unless otherwise specified, the gearbox noise levels shall not exceed 85 dBA at 1 m when operated at rated speed under fully loaded conditions. See SES S20-G01.
2.1.5 Substitute the requirement as follows:
Equipment shall be designed to run safely to the trip speed setting indicated in Table I. Rotors for turbine driven gears shall be designed to operate safely at momentary speeds up to 130 percent of the rated speed.

Table I – Driver Trip Speeds

Equipment driven by induction motors shall be rated at the actual motor speed for the rated condition.
2.1.7 Substitute the requirement as follows:

Motor, electrical components and electrical installations shall be suitable for the area classification (class, group and division or zone) specified by SABIC, and shall meet the requirements of applicable sections of NFPA 70, Articles 500, 501, 502 and 504), and SES E02-E02. In addition, the requirements of SES E19-S01 shall apply.
2.1.11 Add the following sentences:

Unless otherwise specified, gear boxes and auxiliaries shall be designed for operation in unprotected outdoor desert environment. Site specific data shall be used for the equipment design.

2.2 Rating

2.2.1 Gear Rated Power
2.2.1.1 Add the following new paragraph:
For electric motor drivers, the gear shall also be rated to withstand momentary torque overloads equal to
or exceeding four times (4X) gear power rating, to provide for peak torque resulting from reapplication of
voltage after a power interruption. In addition, for synchronous motor drives or variable frequency
induction motor drives, the gear unit including the gear teeth and bearing metal shall be rated to withstand
the peak oscillatory torque values predicted by the system transient torsional analysis of the motor starting
characteristics.

2.2.1.2 Add the following new paragraph:
Vendor shall confirm for optimal design, for example rotor dynamics and lead modification, the
requirements for normal transmitted power in addition to the gear rated power.

2.2.2 Gear Service Factor
Substitute Table 2 as follows:
Table II – Minimum Gear Service Factors

2.2.3.6 Add the following sentence:
See Appendix H for further discussion.

2.3 Casings

2.3.1.3 Substitute the requirement as follows:
Mounting surfaces shall meet the following criteria:
a. They shall be machined to a finish of 3.2 µm (125 microinches) average roughness (Ra) or better.
b. To prevent a soft foot, they shall be in the same horizontal plane within 25 µm (0.001 in)
c. The upper machined or spot faced surface shall be parallel to the mounting surface

Hold down bolts shall be drilled perpendicular to the mounting surface or surfaces, machined or spot faced to a diameter three times that of the hole and, to allow for equipment alignment, shall be 15 mm (1/2 in) larger than the hold down bolt. Pilot holes shall be provided in each foot for field doweling.

2.4 Casing Connections

2.4.3 The decision is as follows:
A nitrogen purge connection shall be provided where the gear is not connected to force feed oil system.

2.5 Gear Elements

2.5.1 General

Unless otherwise specified, gear elements shall be double helical, and gear mesh shall be apex leading.

2.5.2 Quality Assurance
2.5.2.1 Substitute the requirement as follows:
After the gear teeth are finish-cut, shaved or ground on a hobbing, shaving or grinding machine, the gear elements shall be checked in accordance with ANSI/AGMA ISO 1328 for gear tooth accuracy. The gear elements shall have an accuracy of Grade 4 or better. The records of the gear accuracy shall be maintained by vendor for a period of not less than 20 years, and shall be available to SABIC on request.

Note: Previous journal runout chart requirement is replaced with gear charts requirement. ANSI/AGMA ISO 1328 references the journal in the measurement of gear teeth. See also ISO/TR 10064-1 on measurement methods.

Contact tapes shall be provided to SABIC for main and spare rotor sets.
2.5.2.4 Add the following new paragraph:
Gears, shafts at bearings and bearing housings shall be demagnetized to a level of 2-3 gauss. Casings shall be demagnetized to a level of 10-12 gauss.

2.5.3 Fabrication

Pitch line velocities of 150 m/s (30,000 ft/min) shall not be exceeded under any circumstances. Gear manufacturer shall consider the need for lead modifications with pitch line velocities in excess of 110 m/s (22,000 ft/min).

2.5.4 Shafts

2.5.4.1 Substitute the first sentence as follows:
Replace AGMA 6010 reference with ANSI/AGMA 6001-D97.

2.5.4.3.1 The decision is as follows:

Provision shall be made to mount a torsiograph on the low speed shaft, in accordance with Figure 5. Shaft ends where coupling hubs will be mounted shall conform to SES G12-S01. Unless otherwise specified, shafts shall be provided with integral flanges for couplings.

2.6 Dynamics

2.6.1 Critical Speeds

In the design of rotor-bearing systems, consideration shall be given to all potential sources of periodic forcing phenomena (excitation) which shall include the following sources:

a. Unbalance in the rotor system
b. Oil-film instabilities (whirl)
c. Internal rubs
d. Gear-tooth meshing and side bands
e. Coupling misalignment
f. Loose rotor-system components
g. Hysteretic and friction whirl
h. Asynchronous whirl
i. Electrical line frequency

Note 1: The frequency of a potential source of excitation may be less than, equal to, or greater than the rotational speed of the rotor.

Note 2: When the frequency of a periodic forcing phenomenon (excitation) applied to a
rotor-bearing-support system coincides with a natural frequency of that system, the system will be in a state of resonance. A rotor-bearing-support-system in resonance may have the magnitude of its normal vibration amplified. The magnitude of amplification and, in the case of critical speeds, the rate of change of the phase-angle with respect to speed, are related to the amount of damping in the system.

2.7 Bearings

2.7.4 Bearing Housings

Provision shall be made for mounting the following:
a. 1 one event per revolution probe at input and output
b. 2 axial probes at each thrust bearing
c. 1 axial probe on any shaft without a thrust bearing
d. 2 radial probes per radial bearing
e. 2 accelerometers: 1 input and 1 output on the coupling ends
Unless otherwise specified, the following shall be provided:
a. 1 one event per revolution probe at input and output (Total = 2)
b. 2 axial probes at each thrust bearing (Total = 2)
c. 2 radial probes per radial bearing, coupling ends only (Total = 4)
d. 2 accelerometers – 1 per coupling end (Total = 2)

The probe installation shall be as specified in SES G19-S01.
Note: The number and position of axial probes shall consider the type of gear (double or single helical) and thrust bearing location.

2.8 Lubrication

2.8.5 Add the following sentence:
In addition, the requirements of SES G03-S01 shall apply.

2.9 Materials General

Materials used in gear and pinion teeth shall be forged or hot rolled alloy steel of high quality, selected to meet the criteria for tooth pitting index and strength specified in 2.2. In selecting the material, vendor shall consider whether the gear and pinion are to be through hardened, case hardened, or nitrided.

The material quality of gear teeth will conform to ISO 6336-5 1996, material quality grade ME, for case hardened or nitrided steels, and quality grade MX for through hardened steels. The material and manufacturing method shall require SABIC approval.

2.9.3 Heat Treatment

After through hardened gear materials have been rough machined to the approximate final contour of the blank, and heat treated, the tooth area shall be checked for proper hardness. After surface hardened gear materials have been completely heat treated, the surface hardness and case depth shall be checked on a representative coupon of suitable size that has accompanied the part during all heat treat processes.

2.10 Nameplates and Rotation Arrows

Nameplates and rotation arrows shall be of Series 300 stainless steel or of nickel-copper alloy, attached by pins of similar material, and located for easy visibility. As a minimum, the following data shall be clearly stamped on the nameplate:

a. Vendor’s name
b. Size and type of the gear unit.
c. Gear ratio
d. Serial number
e. Gear rated power
f. Rated input speed in rpm
g. Rated output speed in rpm
h. Gear service factor as defined in this standard
i. SABIC item number
j. Number of gear teeth
k. Number of pinion teeth
l. First lateral critical speed of the pinion
m. First lateral critical speed o the gear
SI units shall be shown.

Section 3 – Gear Boxes Accessories

3.2 Couplings and Guards

Dry type flexible disk or diaphragm type couplings shall be provided as specified on the data sheets. Substitute the last sentence as follows:

When an integrally flanged coupling hub shaft end is not furnished, the coupling hubs shall be mounted on the gear shaft by the gear manufacturer, unless otherwise specified.

3.2.2 Substitute ‘API 671’ with ‘SES G12-S01’.

3.2.4 Substitute ‘API 671’ with ‘SES G12-S01’.
3.2.6 Substitute ‘API 671’ with ‘SES G12-S01’.

3.3 Mounting Plates

There shall be 6.35 mm (1/4 in) diametral clearance between the anchor bolts and the anchor bolt holes in the mounting plate.
All baseplate mounting surfaces:
a. Shall be machined after the baseplate is fabricated
b. In the same horizontal plane shall be within 25µm (0.001 in), to prevent a soft foot
c. With different mounting planes shall be parallel to each other within 50µm (0.002 in).

The above tolerances shall be recorded and verified by inspection in unrestrained condition on a flat machined surface at the place of manufacture.
Note: The surfaces being discussed are those on which the equipment is mounted, and on the bottom of the baseplate.

3.4 Controls and Instrumentation used in Gear Boxes

3.4.1 General
3.4.1.1 Add the following sentence:
In addition, the requirements of SES X01-S03 shall apply.
3.4.2.5 Substitute ‘API 670 with ‘SES G19-S01’.
3.4.2.6 Substitute ‘API 670 with ‘SES G19-S01’.
3.4.2.7 Add the following new paragraph:
Unless otherwise specified, thrust and radial bearings shall be fitted with bearing metal temperature
detectors. The temperature detectors shall be installed and calibrated in accordance with SES G19-S01.

3.5 Piping and Appurtenances

Substitute ‘API 614’ with ‘SES G03-S01’.

Section 4 – Inspection, Testing and Preparation for Shipment

4.2 Inspection General

Hardness testing is required as specified.

4.3 Testing

Acceptance of shop tests shall not constitute a waiver to conformance to field tests under specified operating conditions, or relieve vendor from responsibility for conformance to any requirements.

The vibration amplitude/frequency sweep shall also be conducted at minimum operating speed.

To verify the lateral critical speeds, the speed shall be momentarily increased to 120 percent of maximum continuous speed, and phaser amplitude plots shall be made as the unit coasts down to 10 percent of maximum continuous speed.

Note: The bearing temperatures may rise measurably during the test.

Vibration plots shall be required.

Lube oil temperature and pressure variation while testing shall be required.

Tape recordings shall be required.

4.3.3.5 Sound Level Test

The sound level test shall be performed in accordance with ANSI/AGMA 6025-D98 or other agreed standard.

4.4 Preparation for Shipment

The rated capacity of lifting lugs shall be designed for the entire weight of the assembled gearbox plus mounted appurtenances times 2.0 load factor. The lifting lug component material stresses resulting from the rated capacity times 2.0 load factor shall not exceed the allowable stresses for the material specified in the ASME Pressure Vessel Code. The rated capacity less load factor of each lifting lug shall be stamped on the lifting lug.

Section 5 – Vendor’s Data

In this section as engineers or design manager you need to consider vendors data for gear boxes selection and performances.

5.1 General

As a minimum the following is required in addition to the requirements defined in API 613:

a. A General Arrangement Drawing with dimensions
b. A Cross Section Drawing with clearances and backlash
c. A photo of gear tooth contact profile
d. Dimensioned rotor drawings

Appendix H – Gear Tooth Inspection

H.1 General

H.1.1 ANSI/AGMA ISO 1328-1 Cylindrical Gears – ISO System of Accuracy – Part 1: Definitions and Allowable Values of Deviations Relevant to Corresponding Flanks of Gear Teeth and ISO/TR 10064-1. Recommendations relative to gear blanks, shaft center distance and parallelism of axes, describes gear measuring methods and is a general summary of the different procedures used. Many of these procedures may not be applicable due to manufacturing methods and measuring equipment available.

H.1.2 Gears meeting API 613 usually have large diameters and wide face width, and by necessity are manufactured in matched sets with the tolerances in terms of mismatch between the contacting tooth surfaces.
H.1.3 Limits on deviations given in ANSI/AGMA ISO 1328-1 apply to each element and do not cover gears pairs as such. Actual matched gear set accuracy depends on the mismatch as discussed in H.1.2.

Individual elements may meet the accuracy requirements and still not be suitable for service. Conversely, individual elements may not meet accuracy requirements, but with favorable mismatch, the gear set may be suitable. Vendor and SABIC shall discuss the actual acceptance values for each element to achieve the desired gear set accuracy.

H.1.4 The measuring methods described in this appendix cannot be used to replace tooth contact
checking procedures used to verify the gear tooth fit in the job casing in vendor’s shop as described in 2.5.2.2, and the contact checking after field installation and alignment.

H.2 Double Helical Gears

The measuring methods described in ANSI/AGMA ISO standard 2000 1328 and ISO/TR 10064-1 are for single helical or spur gears.

H.2.1 Double helical gears are two single helical gears on the same rotor. These gears shall require data for both the right-hand and left-hand helixes.
H.2.2 When performing tooth contact inspections, it is necessary that the pinion be allowed to move axially to obtain true tooth contact patterns.
H.2.3 Apex runout measurements shall be required in accordance with 2.5.2.3 for double helical gears.

H.3 Modified Tooth Flanks

H.3.1 Lead Modification

H.3.1.1 Every gear under loaded operating conditions is subject to deformation of the entire rotor, in three ways, bending deflection, torsional windup and thermal distortion of the rotor. All three influence the tooth form under operating conditions.

Increased pitch line velocities especially above 140 m/s or 28,000 fpm, result in considerable greater windage and mesh losses which increase the thermal tooth distortion along the flank of the gear teeth. The two variables which most influence the gearset rating are the center distance and the face width of the rotors. If the gear designer elects to reduce the center distance in an attempt to reduce the pitch line velocity and accordingly the windage and mesh losses, he is expected to recover the rating required by increasing the face width.

Increasing the face width increases the bearing span which may result in a less rigid rotor than one where the support bearings are closer together. At the same time a torsional windup of the gearset rotors also takes place. Longer face widths also increase the thermal distortion of the rotors as the gear lubricant is sprayed onto the rotors and pumped along the rotor flanks to the tooth end(s). The accumulated distortion increases with increased bearing span.

To obtain an even load distribution across the entire face width when transmitting the rated power, the pinion teeth should be manufactured with a lead modification having the shape of the inversion of the combined deflection. The total lead modification is the superposition of the modification for the mechanical deflections, bending and torsional, and the modification for the thermal distortion. The typical shape of such a modified lead is shown in Figure H-1.
Note: A method for reducing, but not eliminating, the effects of thermal distortion is through effective (non-uniform) oil spray distribution.

H.3.1.2 Working flanks provided with such lead modifications shall not be used to align a gear, when checked under no load, if the blue tooth contact pattern is too short. When the contact pattern is too short, the non-working flanks of the pinion and gear should be ground without correction (parallel) and used as a basis for correct alignment of the gear. SABIC and vendor shall agree on the patterns obtained in the housing and test stand.

Note: In cases when the L/d ratio is less than shown in Table 3 and the total lead mismatch is less than the limitations prescribed in para. 2.2.3.5, lead modifications may be made to improve contact pattern under load.

H.3.2 Profile Modification

H.3.2.1 Profile Modification:

To prevent engagement shocks due to tooth bending deformations on the working flanks of the pinion and/or gear, the rotors are manufactured with profile modifications to obtain an even trapezoidal load distribution along the path of contact in the transverse section as shown in Figure H-2.

Figure H-1: Tooth Alignment (Lead) Modification of Helical Pinion

Figure H-2 : Profile Modification

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