1、SAE ARP*KLbl 91 7943725 0500337 4 The Engineering Society =For Avanclrig Mobility -Land Sea Alr and Space, INTERNATIONALw 1 400 Commonwealth Drive, Warrendale, PA 15096 - -0001 AEROSPACE RECOMMEND PRACTICE ED ARP1961 Issued 1991-06-21 I I Submitted for recognition as an American National Standard GE
2、ARBOX - AIRFRAME MOUNTED ACCESSORY FOREWORD Since the advent of 1 ightweight/high thrust jet engines, developed for modern combat aircraft, the trend has been to delete accessory drive pads from engines. Engines are furnished with provisions for driving a power takeoff (PTO) shaft. This has created
3、the requirement for a new PTO driven accessory gearbox to provide drive pads for such items as jet fuel starters, air turbine starters, hydraulic pumps, fuel pumps, and AC generators. This new accessory is referred to as an airframe mounted accessory drive or gearbox (AMAD or AMAG). suffered a pleth
4、ora of costly and time-consuming design and development problems. These problems are mainly due to the transfer of gearbox design and performance responsibility from the engine manufacturer to the airframe and gearbox manufacturers, some of whom lack the indepth expertise to conceive and direct a ti
5、mely, trouble-free AMAD/AMAG program. While involving no new principles of design, it has reaching gear and speeds. Furt her, typi cal , In these This is especially true where input shaft speeds and drive pad speeds are now 17 O00 and 25 O00 rpm, respectively; additional difficulties arise where bea
6、ring loads are also increasing due to higher accessory loads and input shaft acceleration and deceleration at rates up to 5000 rpm/s are thus increasing potential for bearing skid problems. cases, not only are bearing and gearing designs demanding but their integrat on into the gearbox design as a w
7、hole (particularly with regard to lubrication and control of increased heat rejection) is a task not to be underestimated. SAE Technical Board Rules provide that: “This report is published by SAE to advance the state of technical and engineering sciences. The use of this report is entirely voluntary
8、, and its applicability and suitability for any particular use, including any patent infringement arising therefrom, is the sole responsibility of the user.“ SAE reviews each technical report at least every five years at which time it may be reaffirmed, revised, or cancelled. SAE invites your writte
9、n comments and suggestions. Cop right 1991 Society of Automotive Engineers, Inc. All rkhts reserved. B Printed in U.S.A. Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE ARP*19bL 91 = 7943725 050033
10、8 b = SAE ARP1961 FOREWORD (Continued) This SAE Aerospace Recommended Practice (ARP) gives broad coverage to the many areas of gearbox design, and provides to the engineer and management a compilation of speci al ized recommendations as we1 1 as program guidance for AMAD/AMAG projects. 1. SCOPE: Thi
11、s SAE Aerospace Recommended Practice (ARP) addresses the characteristics required for the definition, development, and acquisition of a satisfactory airframe mounted accessory gearbox (AMAG) . 2. REFERENCES: MIL-G-6641 (ASG) Gearbox, Aircraft Accessory Drive, General Specification 3. DISCUSSION: A s
12、atisfactory AMAG is defined as a gearbox that will accommodate the usual accessories, i .e., engine starter, %generator, hydraulic pump(s), power transmission shaft, and other accessories as required. It should have low weight, high reliability, and lowest possible heat rejection for a given load. R
13、easonable cost and long life between overhaul are further objectives. Consideration should be given to designing the gearbox such that it could be overhauled on condition versus on a time replacement basis. 4, REQUIREMENTS: The gearbox should meet the general requirements of MIL-G-6641 (ASG), howeve
14、r, exceptions to this specification should be considered where beneficial to the design (one example is where the engine specification attitude 1 imitation conditions should be given preference because they are more rigorous than MIL-G-6641). 4.1 Gearbox Hous i ng : 4.1.1 Materials: Preferable mater
15、ials for 1 and based aircraft gearbox housings are magnesium alloy castings AZ91C-T6 and AZ92A-T6; ZE41A-T5 may be preferred in high temperature applications. aluminum alloy casting A356-T6 is preferred due to its greater resistance to corrosion. standard. For Navy or seaborne aircraft, A casting wa
16、ll thickness of .187 in is considered a good 4.1.2 Supports: Gearbox support points should be three in number to prevent airframe and thermal deflections from imposing undue loads on the housing. Also for this reason, only one support point should resist transverse loading. than necessary. Internal
17、access should preferably be through drive flange openings only, to preserve maximum stiffness in the housing assembly. 4.1.3 Access: Effort should be made to avoid designs with access covers larger -2- Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or
18、 networking permitted without license from IHS-,-SAE ARP*1961 91 7943725 0500339 8 = SAE ARP1961 4.1.4 4.1.5 4.1.6 4.1.7 4.1.8 4.1.9 Bottom Design: sloped toward the point selected for scavenge pump oil pickup. Dry Sump: A dry sump housing design with the oil stored in an integral (or separate) rese
19、rvoir is recommended over a wet sump design where the oil is stored in the bottom of the gear cases. The latter design results in high windage and churning losses by the gears. Internal Oil Tank: housing for an integral oil tank, if possible. space for 10% aeration and 10% expansion (due to heat) of
20、 the gross volume of oil in the tank, lines, and oil cooler. Housing Design: (approximately 4 in outside) to allow oil from bearings and gear oil jets to escape readily and return to the sumping area for pickup by the scavenge pump. Crowded gears, shafts, and bearings result in oil gulping or blocka
21、ge due to high windage and high velocity oil particles. design must provide good oil circulation to and drainage from all bearings. Bearing Housing: Dimensions and tolerances of finished bearing liner bores in the housing must be analyzed to assure that bearing lockup will not occur due to different
22、ial metal contraction during cold weather (e.g. , -65 OF) start-up. In this analysis, include bearing internal clearances as well as housing bore and shaft fits and tolerances, corrected for temperature. distances should also be considered. If possible, the bottom of the gearbox housing should be Fo
23、r dry sump designs, dedicate space within the gearbox The oil tank should allow Housing width (front to back) should be ample Housing The impact of maximum elevated temperatures on gear center Gearshafts: bearing inner race stretch to avoid thermal runaway problems at peak gearbox temperatures. Axia
24、l cl amping and centri fugal effects on the bearing inner races should be accounted for in the analysis. Dimensions and tolerances of gearshafts must provide for 4.1.10 Oil Passages: Oil passages should be cored into the casting where possible. leaks and fatigue failures, which can occur when lines
25、are stressed to com ensate for mismatch. the manufacturer. Separate lines should be avoided to reduce the probability of Flow areas of scavenge passages must be adequate to R andle the mixed air and oil flow. Oil passages shall be cleanable at Oil passages must be 100% proof pressure tested at press
26、ures 150% of maximum setting of system overpressure bypass re1 ief valves. 4.1.11 Oil Discharge: In the layout design of the oil distribution system careful attention should be given to assure that oil flow from regulating and re1 ief valves, ,plus that from scavenge pumps, is discharged into the oi
27、l reservoir or oil tank and not into the gear case cavity. Discharge into the gear cavity would aggravate oil churning and cause unwarranted heat rejection. In the case of wet sump designs, the oil should be discharged in a manner to avoid moving parts and to the lower portion of the gearbox such th
28、at it will generate as little turbulence as possible. -3- Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE ARPUL761 71 7743725 0500340 4 SAE ARP1961 4.1.12 Vibration: Airframe induced vibration load
29、s imposed on the gearbox and mounting system, including the weights and moments of all initialled accessories, should be included in the initial stages of the gearbox design. these loads must be considered as part of the test demonstration requirements. Requirement for crash load resistance, if requ
30、ired, should be defined by the procuring agency. , Ability of the gearbox to operate satisfactorily while enduring 4.2 Mounting Pads: Accessory mounting pads should be serially numbered with the same number as the gearbox housing and should be position marked as to rotation. prevents mismatch if two
31、 or more gearboxes are assembled or overhauled at the same time in the same area. It also assures pad covers will be in the same position as they were in during line boring, made from A356-T6 aluminum alloy to withstand scuffing by V-band clamps and possible corrosion. If they are made from magnesiu
32、m, corrosion protection would be required. This V-band pad covers should be 4.2.1 4.2.2 4.2.3 4.2.4 4.2.5 Seals and Drainage: Where a mounting pad is provided to drive a fuel pump, review carefully the fuel pump seal , the AMAG oil seal , and pad cavity drainage provisions to assure they are adequat
33、e to drain any escaping fuel. This will prevent a fuel pressure buildup that could cause fuel to enter the AMAG case. Accessory Failure: determine how accessory failure (including shear groove) may affect the integrity of the AMAG. design involve an oil system shared with other components. The failu
34、re mode of gearbox mounted accessories (in the event of a failed bearing or seal in the accessory) should be such that loads transmitted into the gearbox are low enough not to damage the gearbox. The resulting imbalance of the accessory shaft system shall be such that it does not exceed the maximum
35、specified imbalance and duration permitted at the gearbox pad face. Interface Requirements: engagement of pad splines, nipples, pilots, etc. to verify that accessories and interface hardware engage properly. before gaskets and seals are squeezed to avoid damage to the seals and gaskets during assemb
36、ly. At this time, verify that actual rotation corresponds with rotation markings of accessories and their respective mounting pads. sufficient to accommodate an accessory driveshaft, failed in shear at up to a 45O angle. Disengagement/Shear Section: emergency disengagement devices and output shaft s
37、hear sections. Meetings with accessory vendors should be held to This is particularly important should the AMAG Conduct a separate review of the sequence of Splines and pilots must engage Finally, verify that gear shaft female spline depths are Clutch driven accessories should have Spline Clearances
38、: spline and shoulder clearances can accommodate center1 ine offset and angul ar mi sal ignments. Review all AMAGlaccessory spline fits to be sure that Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-S
39、AE ARP*19b1 91 7943725 0500343 b SAE ARP1961 4.2.6 1 4.2.7 Weight and Moment Margins: overhung moments should be specified to have significant margins to accommodate growth and/or accessories that exceed their design requirements in these areas. Mounting pad design must be adequate to withstand stat
40、ic and dynamic 1 oadi ng . Generator Load Growth: gearbox housing with sufficient extra strength to accommodate and support a larger generator, should the electrical loads grow in magnitude after the airplane is in service. alternator is not unusual. configuration and strength can be changed at a la
41、ter date with little difficulty. This recommendation is made since electrical load growth occurs during the service life of most aircraft whether they are commercial or military. Mounting pad values for accessory weights and Experience has shown that this is a frequent occurrence. Consideration shou
42、ld be given to designing the For example, growth from a 40 kVA to a 60 kVA Gear strength and generator mounting pad 4.3 4.3.1 Critical Speed Margin: Stiffness of the driveshaft, bearings and gearbox housing must be such that the connecting PTO shaft between the engine and gearbox will be free of cri
43、tical speed vibrations up to at least 125% of maximum engine speeds, including transient overspeeds defined by the engine specification. This element of the gearbox design should receive prompt attention and be coordinated (jointly if possible) with the shaft, engine, and gearbox manufacturers in th
44、e earliest phase of gearbox design. survey of an installed PTO shaft should be conducted as early in the program as possible. undesirable add on fixes. Gearbox Dri veshaft/Housi ng St i ffness : A critical speed Failure to do this may result in costly redesign or 4.3.2 Case Deflection: A case deflec
45、tion analysis should be made to assure that deflections due to internal air pressure will not result in bearing outer race misal ignment beyond that stipulated by bearing manufacturers. extent of case deflection will be governed by the type of venting system selected by the designer. Internal case p
46、ressure should be controlled by suction and pressure re1 i ef val ves. bearings and seal, the gearbox should be considered. The 4.3.3 Accessory Heat Loads: Many starters continue to rotate on output shaft Heat load generated by accessories, soaking back into 4.4 Gearbox Driveshaft: 4.4.1 Splined Sha
47、ft: The shaft should be splined to permit adequate fore and aft displacement to accommodate airframe/engine tolerances and deflections including effects of flight loads and thermal expansion. It is advisable to harden the mating surfaces of the coupling shaft and input shaft to inhibit fretting and
48、wear resulting in shaft looseness and consequential lowering of PTO shaft critical speed. 4.4.2 Hardened Mating Surfaces: -5- Copyright SAE International Provided by IHS under license with SAENot for ResaleNo reproduction or networking permitted without license from IHS-,-SAE ARP*LSbL 91 7943725 050
49、0342 B SAE ARP1961 4.4.3 Spline Lubrication: 4.5 Gearing: 4.5.1 Location: Accessory splines should be lubricated by oil jet to assure long life; an O-ring on the spline shaft will prevent leakage. Gears should be located as high in the housing as feasible to reduce oil churning. with wet sumps. This is especially important for gearboxes designed 4.5.2 Shrouds: Thin gears such as oil pump drive gears should be given lower priority regarding location, but, if they extend towards the lower area of the ho
