Sep 1, 2014

Determining Gear Efficiency

By analyzing certain parameters, the authors present an original method for determining the efficiency of gears.


Abstract: This paper presents an original method for determining the efficiency of the gear. The originality of this method relies on the eliminated friction modulus. In the following pages we analyze the influence of a few parameters concerning gear efficiency. These parameters are: z1—the number of teeth for the primary wheel of gear; z2—the number of teeth of the secondary wheel of gear; αo—the normal pressure angle on the divided circle; and β—the inclination angle. With the relations presented in this paper, one can synthesize the gear’s mechanisms. Today, gears are present everywhere in the mechanical world, especially the automotive, electronics, and energy industries, etc. By optimizing this mechanism, one can improve the functionality of transmission gears.

Introduction
In presenting this original method for calculating the efficiency of the gear, the originality consists in the way of determining the gear’s efficiency, because we haven’t used the friction forces involved in coupling—this new way eliminates the classical method, in fact. The necessity of determining the friction coefficients by different experimental methods is eliminated, as well. The efficiency determinates by the new method are the same as in the classical method; namely, the mechanical efficiency of the gear. Precisely, one determines the dynamics efficiency, but at the transmission’s gears, the dynamics efficiency is the same as the mechanical efficiency.

Determining the Momentary Dynamic (Mechanical) Efficiency
The calculating relations [2, 3], are the next (1-21), (see fig. 1):

(1)

with: Fm - the motive force (the driving force);
Fτ - the transmitted force (the useful force);
Fψ - the slide force (the lost force);
v1- the velocity of element 1, or the speed of wheel 1 (the driving wheel);
v2- the velocity of element 2, or the speed of wheel 2 (the driven wheel);
v12- the relative speed of the wheel 1 in relation with the wheel 2 (this is a sliding speed).

Figure 1

The consumed power (in this case the driving power):
(2)
The useful power (the transmitted power from the profile 1 to the profile 2) will be written:
(3)
The lost power will be written:
(4)

The momentary efficiency of couple will be calculated directly with the next relation:
(5)
The momentary losing coefficient [1], will be written:
(6)

One can easily see that the sum of the momentary efficiency and the momentary losing coefficient Now one can determine the geometrical elements of the gear. These elements will be used in determining the couple efficiency, η.

The Geometrical Elements of the Gear
One determines the next geometrical elements of the external gear, [2,3], (for the right teeth, β=0):
The radius of the basic circle of wheel 1 (of the driving wheel), (7):
(7)
The radius of the outside circle of wheel 1 (8):

(8)

One determines now the maximum pressure angle of the gear (9):

(9)

 

And now one determines the same parameters for the wheel 2, the radius of basic circle (10) and the radius of the outside circle (11) for the wheel 2:
(10) (11)

Now one can determine the minimum pressure angle of the external gear (12, 13):
(12)

(13)
Now one can determine, for the external gear, the minimum (13) and the maximum (9) pressure angle for the right teeth. For the external gear with bended teeth (β≠0) one uses the relations (14, 15 and 16):
(14) (15) (16)

For the internal gear with bended teeth (β≠0) one uses the relations (14 with 17, 18-A or with 19, 20-B):

A. When the driving wheel 1 has external teeth:
(17) (18)

B. When the driving wheel 1 has internal teeth:

(19)

(20)

 

Determining the Efficiency
The efficiency of the gear will be calculated through the integration of momentary efficiency on all sections of gearing movement, namely from the minimum pressure angle to the maximum pressure angle, the relation (21), [2, 3]:
(21)
"More precise (5) for determining momentary efficiency is the relation (22):
(5) (22)

Conclusion
The input parameters are: z1 = the number of teeth for the driving wheel 1;
z2 = the number of teeth for the driven wheel 2, or the ratio of transmission, i (i12=-z2/z1);
α0 = the pressure angle normal on the divided circle;
β = the bend angle.

The efficiency of the gear increases when the number of teeth for the driving wheel 1, z1, increases too, and when the pressure angle, α0, diminishes. In addition, z2 or i12 are not much influence on the efficiency value.

One can easily see that for the value α0=20o, the efficiency takes roughly the value η≈0.89 for any values of the others parameters (this justifies the choice of this value, α0=20o, for the standard pressure angle of reference). The better efficiency may be obtained only for a α0≠20o, the efficiency takes roughly the value η≈0.89 for any values of the other parameters (this justifies the choice of this value,α0 =20o, for the standard pressure angle of reference). The better efficiency may be obtained only for a α0 ≠20o. But the pressure angle of reference, α0, can be decreased at the same time the number of teeth for the driving wheel 1, z1, increases, to increase the gears’ efficiency.

The module of the gear, m, has no influence on the gear’s efficiency value. When α0 is diminished one can take a higher normal module for increasing the addendum of teeth, but the increase of the m at the same time with the increase of the z1 can lead to a greater gauge. The gears’efficiency, η is really a function of α0 and z1: η =f(α0,z1); αm and αM are just the intermediate parameters. For good projection of the gear it’s necessary to have a z1 and z2 greater than 30-60, but this condition may increase the gauge of mechanism.

In this paper we have discussed how one determines the dynamics efficiency, but at the transmission’s gears the dynamics efficiency is the same as the mechanical efficiency. This is a greater advantage of the transmission’s gears. This advantage of the gear’s mechanisms may be found at the cam’s mechanisms with plate followers, as well.

References:
1 Pelecudi, Chr., ?.a., Mecaisme. E.D.P., Bucure?ti, 1985.
2. Petrescu, V., Petrescu, I., Randamentul cuplei superioare de la angrenajele cu roti din? ate cu axe fixe, In: The Proceedings of 7th National Symposium PRASIC, Bra?ov, Romania, vol. I, pp. 333-338, 2002.
3. Petrescu, R., Petrescu, F., The gear synthesis with the best efficiency, In: The Proceedings of ESFA’03, Buchare?t, Romania, vol. 2, pp. 63-70, 2003.
4. Petrescu, R., Petrescu, F., Gear’s design for the mechanism’s highest efficiency. In: The Proceedings of ICEGD’05, Bucharest, Romania, vol. I, pp. 257-260, 2005.
5. Petrescu, R.V., Petrescu, F.I., Determining the dynamic efficiency of gears. In: The Proceedings of SYROM’05, Bucharest, Romania, vol. II, pp. 381-386, 2005.

About The Authors
Relly Victoria Petrescu is a lecturer in the Department of Descriptive Geometry and Engineering Graphics at the University Politehnica of Bucharest.
Narcisa Popescu is an associate professor in the Department of Descriptive Geometry and Engineering Graphics at the University Politehnica of Bucharest.
Florian Ion Petrescu is an assistant professor in the university’s in Department of Mechanisms and Robots.

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