Precision forging straight and helical spur gears

Journal of

ELSEVIER

J. Mater. Process. Technol. 45 (1994) 25-30

Materials Processing Technology

P r e c i s i o n F o r g i n g S t r a i g h t and H e l i c a l S p u r G e a r s M.H. Sadeghi a and T.A. Dean b a Dept. of Mechanical Engineering, Tarbiat Modarres University & Press Iran Co. Tehran, Iran b School of M a n u f a c t u r i n g and M e c h a n i c a l E n g i n e e r i n g & IRC. for Mat er i al s for High Performance Applications, Birmingham University, U.K. Aspects of the precision forging straight and helical spur gears including die design, preform design, dimensional accuracy of tooth forms, forging loads and ejection problems are reviewed. The effect of gear geometry and process variables on forging pressure and ejection load are discussed. Based upon results of experiments performed by the authors and a literature review, proposals are made for designing preforms for precision forging simple and compound straight and helical spur gears. Experimentally obtained traces of forged teeth are compared with theoretical profiles of the corresponding die and final forging. 1. INTRODUCTION Usually, gears are produced either by shaping or bobbing. Due to the complex contours and high accuracy requirements of the gear teeth, gear m a n u f a c t u r i n g is highly specialized, inherently demanding of machining time and t h e r e f o r e costly. M a t e r i a l wastage is also another problem in these processes. In recent years forming techniques have been i n t r o d u c e d as a l t e r n a t i v e p r o d u c t i o n routes. These include fine blanking, press shaving, roll forming and precision forging. The aim of these processes is to produce gears near to finished form. Fine blanking and press shaving is limited to gears of up to about 6 mm width (although one report [1] suggests thicker gears can be made by warm shaving). Material yield and mechanical properties are not significantly improved compared to conventional means. Roll forming has a slow production rate [2] and often can not be justified by cost. It is also limited to ring gears while precision forging can be used for variety of sizes of solid or hollow gears.

In recent years, there has been increased interest in the production of gears by precision forging. Precision forging enables gear teeth to be m a n u f a c t u r e d to nett or near nett tolerances, resulting in significant savings in raw material and production time compared with c o n v e n t i o n a l cutting methods. Improved mechanical properties and if cold coining is used, surface finish of the finished part and higher production rates are added advantages. Furthermore, this a p p r o a c h to g e a r m a n u f a c t u r e is less d e p e n d e n t on the availability of specialized machine tools and skilled labour required to set up and operate them compared with c o n v e n t i o n a l cutting methods. Due to the high cost of tooling, precision forging of gears is unlikely to be profitable for small volume production. High volume production makes the precision forging of gears, an attractive production route for automotive industries. In c o n t r a s t to the above advantages, precision gear forging is associated with

0924-0136/94/$07.00 © 1994 - Elsevier Science B.V. All rights reserved. SSD1 0924-0136(94)00155-3

26

}:~'..i:,;i"i:?:,,7:~;71'{ punch

punch container

_ _

~r-7-n container ' d" ,' "

bolster , '~ ~/ " . "",

i

~" "'" ' '~ """

(a) fixed container

" ,;

.... i'..'"", "' . . . . . . . .

f,. j

,e ÷,

".',.,"...".'. b~)Islel

(b) movable containm

Figure 1. Die-set designs liar forging spur and helical gears. problems related to die design, preform volume and geometry, tooth dimensional accuracy, load a n d e n e r g y p r e d i c t i o n and f i n a l l y e j e c t i o n problems. These are reviewed in this paper.

2. DIE DESIGN in Fig.1. two basic die-set designs for forging s p u r or h e l i c a l gears are s h o w n . In b o t h alternatives female forms of the gear teeth are machined in the c o n t a i n e r bore. The anvil has teeth m a c h i n e d r o u n d its periphery and is a close fit in the c o n t a i n e r . The die is thus scraped clean when the anvil moves upwards to eject each forging. l n d e s i g n (a) the only moving part is the punch which enters the c o n t a i n e r to deform t h e w o r k p i e c e . F o r f l a s h l e s s f o r g i n g the periphery of the punch must be gear shaped as is the anvil. The punch in design (b) does not enter the c o n t a i n e r b u t closes the cavity by p u s h i n g it against light s u s p e n s i o n springs, The punch does not need to be accurately machined to fit the container and in general can be made more cheaply than that in design (a). The general mode of cavity filling in design (a) is the reverse of that for design (b).

3. D E S I G N OF P R E F O R M V O L U M E AND GEOMETRY As in any other precision lorging process, exact control of billel volume is a problem in precision gear forging. This problem has been discussed by different investigators including Dean [31, M e y e r - N o l k e m p e r [4] and Nediani [5]. O p t i m u m p r e f o r m g e o m e t r y is a n o t h e r p r o b l e m to be dealt with. 111 order t o o b l a i n defect free gears with adequately filled corners, the correct p r e f o r m g e o m e t r y is necessary° Furthermore, preform geometry influences the forging load r e q u i r e m e n t s to fill a die cavily. T h e r e is little q u a n t i t a t i v e i n t ' o r m a t i o n on p r i n c i p l e s of p r e f o r m design for precision forging in the literature. Here, based upon the r e s u l t s of e x p e r i m e n t s p e r f o r m e d by the authors [6] and literature review, the following design p r o p o s a l s have b e e n layed down for designing preforms for precision forging straight or helical spur gears. * The volume of the preform should be equal to that of the final gear volume and should be closely c o n t r o l l e d in o r d e r ~o avoid underfilling or overstressing of tools. * T h e g e o m e t r y of p r e f o r m s s h o u l d be designed in such a w a y that an u p s e t t i n g mode of flow predominates. This gives beUer die filling and less load requirements [7,8]. * In elevated temperature forging, if contact

27

,Forged Tooth Profile

B~,eCi.'e

I

~1 ~"

/

I

B..~Cir¢'e

Forged Too-'th Proflh (Experiment)

Figure 2. S u p e r i m p o s i t i o n of the experimentally obtained profile of a forged tooth on theoretical profiles of the corresponding die and final forging, for forging at 1000°C temperature

flow i n t o t e e t h , s t r o n g e r t e e t h will be p r o d u c e d . T h i s type of flow, is likely to r e d u c e t h e a m o u n t of s l i d i n g a c r o s s t h e female form teeth surface, hence, reduces die wear. For forging hollow gears, ring p r e f o r m s with maximum outer diameter and a clearance between their inner diameter and mandrel, will r e s u l t in t h e m o s t f a v o u r a b l e s u r f a c e strains at the tooth tips. This will help reduce t h e l i k e l i h o o d of s u r f a c e c r a c k i n g at t h e tooth tips during deformation 19].

4. DIMENSIONAL ACCURACY

*

*

*

*

*

with t h e side walls of the die occurs b e f o r e the c o r n e r s are filled, t h e c o m b i n a t i o n of increased frictional resistance and increased yield strength through cooling, makes filling of the corners extremely difficult. Therefore, the geometry of preform should be designed to delay side wall contact. F o r f o r g i n g h o l l o w g e a r s , use of h o l l o w preforms are most advantageous. The g e o m e t r y of t h e h o l l o w p r e f o r m s h o u l d be designed for outward flow and minimum side wall contact (i.e. minimum amount of sliding across female form of gear teeth of die). F o r good die filling, t h e p r e f o r m g e o m e t r y s h o u l d not p r o d u c e fin u n t i l t h e very end of the forging operation. In case of u s i n g c y l i n d r i c a l p r e f o r m s for d o u b l e spur or helical gear, for a given boss diameter (root diameter of smaller gear), the smaller the outside diameter of the preform, the easier is the filling of the boss. In case of cylindrical preforms, the maximum o u t s i d e d i a m e t e r of the p r e f o r m s h o u l d be equal to root diameter of the smaller gear in o r d e r to e l i m i n a t e free u p s e t t i n g stage and possible folding over problems. if p r e f o r m g e o m e t r y is d e s i g n e d for radial

In precision gear forging the ultimate aim is to p r o d u c e gears with i n t e g r a l t e e t h w i t h o u t t h e n e e d for a n y p o s t - f o r g i n g m a c h i n i n g operations. This requires design and manufacture of dies with consistent accuracy. In p r e c i s i o n gear forging, t h e r e m u s t be s m a l l differences between the dimensions of the gear shaped die cavity and the forged gear. This is to compensate for the following changes: (i) U n d e r t h e f o r g i n g l o a d s , die c a v i t i e s e x p a n d elastically, t h u s its initial d i m e n s i o n s must be made smaller. (ii) If t h e w o r k p i e c e is at a n e l e v a t e d t e m p e r a t u r e , dies are p r e h e a t e d and t h e r e f o r e expand and the cavity should be made smaller. (iii) Post forging contraction of the c o m p o n e n t occurs at e l e v a t e d t e m p e r a t u r e s therefore, the die cavity must be made larger. (iv) In p r e c i s i o n g e a r f o r g i n g , dies a r e normally made by electro- discharge machining. Due to the s p a r k gap allowance, t h e e l e c t r o d e should be made smaller. H e n c e , to o b t a i n a c c u r a t e forged gears, the e x t e n t of e a c h of t h e a b o v e f a c t o r s has to be calculated and allowed for. Using the values of d i m e n s i o n a l c h a n g e derived previously for solid

28

P

Vndn~--~mm .... hr.~-o.o

!

.-7~ M-~5,i

i

i

/ i I.-i~

i

T-Hi

. . . . i~,=-~.--7

[l' i

i

i

.....

T

.......

i

:

!

F'rl~lon I~a~or--~-O.2

:

j

Jl=~

J

~

~__f.~

,,

/~ ~ -

~

-..*

~d

t~

"

"

i

i

'

~

i

~

,

(

"

;.

"

'

i

~

Figure 3. variation of relative forging pressure with deformation for various number of teeth in forging solid straight spur gears cylinders [10], the above factors affecting final forging dimension can be used to determine the relation between the radius of die and of forging. Assuming every point on a radius of forging moves only along that radius, it has been found that profile of a forged tooth in an involute die t o o t h cavity r e m a i n s i n v o l u t e [ l l ] . Fig.2 compares the experimentally obtained traces of forged teeth at 1000 C with theoretical profiles of the c o r r e s p o n d i n g die and final forging. It has been found that the post-forged profile of a t o o t h forged in an i n v o l u t e die t o o t h cavity remains involute [111.

Figure 4. Variation of ejection load with numbcl of teeth and gear module ti)r lZ)rging spur gears

*

*

*

* 5. LOAD AND ENERGY PREDICTION ~' T h e r e are few d e v e l o p e d load prediction m e t h o d s for p r e c i s i o n gear forging. A theoretical model based on the energy method, for predicting forging load and bulged surface p r o f i l e of the t e e t h when forging s t r a i g h t or helical spur gears has b e e n p r e s e n t e d by the authors peviously [6,12]. It has been found that; * The forging pressure increases considerably and p r o g r e s s i v e l y with d e f o r m a t i o n as the f r i c t i o n f a c t o r increases. The i n c r e a s e is

r e m a r k a b l y higher after the f o r m e d teeth c o n t a c t e d the die wall, i.e. d u r i n g c o r n e r filling stage. For a given gear module and width, Iorging pressure increases by increase in number of teeth (see Fig.3). F o r g i n g p r e s s u r e v a r i e s very l i t t l e with increase in gear modnle during the first stage of d e f o r m a t i o n and before the formed lecth contact the die wall. D u r i n g c o r n e r filling, the pressure increases considerably as the gear module increases. Forging pressure increases very slight ly wit h increase in helix angle over the entire range of deformation. For a given gear size, the thinner the gear, the higher the forging load requirements. Forging pressure increases with increase in b o r e d i a m e t e r and this i n c r e a s e in m o r e considerable during corner filling stage.

6. EJECTION PROBLEMS In c o n v e n t i o n a l d r o p f o r g i n g in o r d e r to enable the workpiece to be removed easily from the die, a draft angle is used. In general, as the draft angle decreases, more force is required to

29

*

(a)

(b)

*

F i g u r e 5. T o o l i n g M i s m a t c h a) P r e c i s i o n i m p r e s s i o n in o n e die half (No mismatch) b) P r e c i s i o n i m p r e s s i o n in b o t h die h a l v e s (Mismatch arises) * e j e c t t h e f o r g i n g f r o m t h e die cavity. In precision forging although some degree of draft angle can be used in preforming to ease ejection, in the final s t a g e usually no draft can be allowed on the component. This makes forged straight and helical spur gears more difficult to eject and distortion of final product may occur. E j e c t i o n of the helical gears b e c o m e s m o r e difficult than s p u r gears due to the helix angle. I n c r e a s e in helix a n g l e causes m o r e frictional resistance during ejection, therefore, making the release of the c o m p o n e n t from the die even more difficult. A t h e o r e t i c a l analysis for predicting loads required to eject straight and helical spur gears has been developed by the authors [13]. It has been found that; * For a constant gear height, the ejection load increases with increase in friction factor. * The higher the forging load, the m o r e the gear cavity filling and t h e r e f o r e , the m o r e the interface contact area, resulting in higher ejection load requirements. * T h e h i g h e r the f o r g i n g t e m p e r a t u r e , the lower the ejection load. * For a spur gear with given module, pressure angle and width, ejection load increases by

*

increase in number of teeth. The increase in e j e c t i o n load with i n c r e a s e in n u m b e r of t e e t h , is g r e a t e r for g e a r s with h i g h e r modules (see Fig.4). For a spur gear with given number of teeth, p r e s s u r e a n g l e and width, e j e c t i o n load increases by increase in gear module. The increase in e j e c t i o n load with increase in gear module, is greater for gears with greater numbers of teeth. For a spur gear with given module, pressure angle and width, ejection load increases with i n c r e a s e in helix angle. T h e g r e a t e r the n u m b e r of teeth, the g r e a t e r the effect of helix angle on ejection load. For a spur gear with given pressure angle and width, n o n - d i m e n s i o n a l e j e c t i o n pressure, decreases with increase in either number of teeth or gear module. The lower the number of teeth, the greater the effect of gear module on e j e c t i o n pressure. Also, the lower the gear module, the greater the effect of number of teeth on ejection pressure. For a spur gear with given module, number of t e e t h , p r e s s u r e a n g l e and w i d t h , the non-dimensional ejection pressure increases, with f r i c t i o n factor and helix angle. This i n c r e a s e is g r e a t e r at h i g h e r f r i c t i o n a l conditions and higher helix angles.

7. OTHER PROBLEMS P r e c i s i o n f o r g i n g of gears w o u l d be economical, if the dies can be designed and produced with consistent accuracy at reasonable cost. Die design can influence the deformation pattern and t h e r e f o r e r e q u i r e d forging load. The dimensional accuracy of final forged gears is also affected by die design. S u r f a c e d e g r a d a t i o n and wear of the die surface and degradation of the workpiece due to oxidation can occur in elevated t e m p e r a t u r e

30

forgings. Wear of the critical surfaces of the die is also a n o t h e r problem. These two factors together with the accuracy required, determine the die impression life and therefore the overall cost of the process. In elevated temperature forgings formation of an oxide film (scale) on the surface of the forging can lead to surface defects. This problem can be c o n t r o l l e d by the use of p r o t e c t i v e a t m o s p h e r e s or low preheat temperatures. When the impression lies in one die-half no die mismatch occurs, but when it is formed in both die-halves (Fig.5), tooling mismatch can arise (i.e. concentric and rotational mismatch). By use of accurate guides concentric mismatch can be reduced and rotational mismatch can be kept low using keys, but depending upon the accuracy required, these measures can be difficult and costly.

8. CONCLUSION Various aspects of the precision forging straight and helical spur gears including die design, preform design, dimensional accuracy of tooth forms, forging loads and ejection problems have been reviewed. Based upon results of experiments performed by the authors [6] and a literature review, some design proposals have been layed down for designing a preform for precision forging straight and helical spur gears and compound gears. Experimentally obtained traces of forged teeth are compared with theoretical profiles of the corresponding die and final forging and it has been found that profile of a forged tooth in an involute die tooth cavity remains involute. Effects of gear geometry and process variables on forging pressure and ejection load are discussed.

REFERENCES 1. M.Murakawa, et al. Production of Thick Spur and Helical Gears by Warm Shaving Proc. of 14th NAMRC, (1986), pp. 445-451. 2. J.B. Hawkyard, et al. Cold Rolling of Ring Gears Proc. of the 15th I n t e r n a t i o n a l M.T.D.R. C o n f e r e n c e , U n i v e r s i t y of Birmingham, (1974) pp.507-513. 3. T.A. Dean The Feasibility of Flashless Forging Metallurgia and Metal Forming, Vol.44, NOS. 11 and 12, NOV. & DEC. 1977, PP. 488-498 and 542-544. 4. H. Meyer-Nolkemper Werzeuge zum form pressure ohme Grat (Tools for flashless forging) Industrie Anzeiger, If)l, JG, N r. 73, 12/9/1979. 5. G . N e d i a n b A n I n t r o d u c t i o n to llashless forging of long shapes Ph.D. Thesis, 1982, Birmingham University. 6. M . H . S a d e g h i ~ P r e c i s i o n Forging Axisymmetric Shapes, Straight and Helical Spur Gears Ph.D. Thesis, 1989, Birmingham University. 7. S.K. Biswas and W.A. K n i g h b C o m p u t e r Aided Design of Axisymmetric Hot Forging Dies Proc.15th Int. M.T.D.R. Conf., 1974, 8. T. Altan, S. Oh and H.L. Gegel, Metal Forming : Fundamentals and Applications, A.S.M. Series in Metal Processing, Ohio, 1983. 9. M. Robinson~A Workability Analysis of the Cold Forging of Gear with Integral Teeth, J. of Mech. Working Tech. 1 (1977/78). 10. M.H. Sadeghi and T.A. Dean "Analysis of Dimmensional Accuracy of Precision Forged Axisymmetric Components" 1~ Mech. E. vol.205 , London, 1991 II.M.H. Sadeghi and T.A. Dean "Analysis of Tooth Profile Accuracy in Precision Forged Spur Gears with Involute Teeth" Proc. Conf. of SME/NAMRC, Vol.21 ,1993 12. M.H. Sadeghi and T.A. Dean "Mathematical Modelling & Experimental Validation of the Precision Forged Straight & Helical Spur G e a r F o r m s " A S M E , 1993 ( s e n t for publication) 13. M.H. Sadeghi and T.A. Dean "The Ejection of Precision Forged Straight & Helical Spur Gear Forms" J. of Materials Processing Technology, vol.31 , 1992