11.1 The Properties of MC Nylon and Duracon
MC is the abbreviation of ‘MONO CAST’. MC is essentially Polyamide 6 (Nylon).
Duracon is a crystalline thermoplastic otherwise called an acetal copolymer. It is one of the most popular engineering plastics.
The characteristics of these plastics are:
- Ability to operate with minimum or no lubrication, due to their inherent lubricity.
- Quietness of operation.
- Lightweight. Excellent resistance to organic chemicals and alkalies.
On the other hand, these plastics are subject to greater dimensional instabilities, due to their larger coefficient of thermal expansion and moisture absorption. These should be taken into consideration when plastic parts are designed. Therefore the design engineer should be familiar with the limitations of plastic gears. It is usual that plastic gears are brought into use after a practical test.
(1) Mechanical Properties
Indicated in Table 11.1 are the mechanical properties under standardized test conditions. In regards to these mechanical properties, the strength tends to become less if the temperature rises.
Table11.1 Mechanical Properties of MC Nylon and Acetal Copolymer
NOTE 1 The data shown in the table are MC Nylon reference values measured at absolute dry.
NOTE 2 For Acetal Copolymer, compressive strength is “(10% deformation point)”.
(2) Thermal Properties
Compared to steel, plastic materials have larger dimensional changes from temperature change. Thermal properties of MC Nylon and Acetal Copolymer are indicated in Table 11.2.
Table11.2 Thermal Properties of MC Nylon and Acetal Copolymer
NOTE 1 The data shown in the table are MC Nylon reference values measured at absolute dry.
NOTE 2 For use in low temperatures, consider the brittle temperature (-30C to -50C degrees) and determine in accordance with your experiences or tests performed.
Calculation Example for the Dimensional Change in a MC Nylon (MC901) Gear Rack
Assumed Product Model No.：PR2-1000 (Total Length: 1010 mm)
Assumed Condition Before Use:
– Atmospheric Temperature: 20 C deg. = Product Temperature: 20 C deg.
– Total Length 1010 mm
Assumed Increase of Temperature
– 20 C deg. to 40 C deg. Rise by 20 C deg.
Coefficient of Linear Thermal Expansion :
– 9×10-5 / C deg.
Calculation Example:
Dimensional Change = Coefficient of Linear Thermal Expansion x
Length x Temperature difference
= 910-5 / C deg. x 1010 mm x 20 C deg. (Temperature difference)
= 1.818 mm
This calculation indicates that a MC Nylon-made PR2-1000 Rack
(Total length: 1010 mm) lengthens by 1.8 mm after a 20 C deg. temperature rise.
(3) Water Absorption Property
Mechanical properties and thermal properties of plastics deteriorate when plastics absorb moisture.
Table 11.3 indicates the water and moisture absorption properties of MC Nylon and Duracon.
Table11.3 Water and moisture absorption properties of MC Nylon and Duracon
NOTE 1 As for 1.MC602ST, the rate of water absorption is approx. 90% of MC901.
Compared with MC Nylon, Duracon has less water absorbing property.
Dimensions of MC nylon gears change with moisture content. This may cause the sizes to vary from the time of purchase to the time of usage. The following figure and the chart show the moisture content and its effect on the dimensions of MC901 Nylon.
Fig.11.1 Moisture Content vs. Dimensional Variation of MC901
Calculation Example for Dimensional Expansion in MC Nylon (MC901) Rack
Assumed Product Model No.：PR2-1000 (Total Length: 1010 mm)
Assumed Conditions Before Use
– With Water Absorption Rate at 1%
– Total Length: 1010 mm
Assumed Conditions after Swelling
Assumed when the water absorption rate increase to 3% at normal temperature.
Calculation Example
1. From the data in Figure 11.1;
– It is determined that the dimensional expansion is 0.2%, as the water absorption rate is 1% before use.
– It is determined that the dimensional expansion is 0.75%, as the water absorbtion rate is 3% after swelling.
2. The increment is calculated as follows:
0.75% – 0.2% = 0.55%
3. As total length is 1010 mm, and the dimensional expansion is determined as below;
1010mm x 0.55% = 5.555mm
(4) Antichemical corrosion property
MC Nylon
Nylon MC901 has almost the same level of antichemical corrosion property as nylon resins. In general, it has a better antiorganic solvent property, but has a weaker antiacid property. The properties are as follows:
– For many non organic acids, even at low concentration at normal temperature, it should not be used without further tests.
– For non organic alkali ar room temperature, it can be used to a certain level of concentration.
– For the solutions of non organic solts, it will be all right to apply them to a fairly high level of temperature and concentration.
– It has better antiacid ability and stability in organic acids than in non organic acids, except for formic acid.
– It is stable at room temperature in organic compounds of ester series and ketone series.
– It is stable at room temperature in aromatics.
– It is also stable in mineral oil, vegitable oil and animal oil, at room temperature.
Table 11.4 lists antichemical corrosion properties of Nylon resin. Please note that the data mentioned might differ depending on usage conditions, so pre-testing should be performed.
Table11.4 Chemical Resistance Properties of MC Nylon
Duracon
One of the outstanding features of Duracon is excellent resistance to organic chemicals and alkalies. However, it has the disadvantage of having a limited number of suitable adhesives. Its main properties are:
– Excellent resistance against inorganic chemicals. However it will be corroded by strong acids such as nitric acid, hydrochloric acid and sulfuric acid.
– Household chemicals, such as synthetic detergents, have almost no effect on Duracon.
– Does not deteriorate even under long term operation in high temperature lubricating oil, except for some additives in high grade lubricants.
– With grease, it behaves the same as with oil lubricants.
In order to acquire knowledge about the resistance against other chemicals, plastic manufacturers’ technical information manuals should be consulted.
11.2 Strength of Plastic Gears
(1) Bending strength of spur gears
MC Nylon
The allowable tangential force F(kgf) at the pitch circle of a MC Nylon spur gear can be obtained from the Lewis formula.
F = mybσb f (kgf) (11.1)
Where
m ：Module(mm)
y ：Tooth profile factor at pitch point (See Table 11.5)
b ：Facewidth(mm)
σb ：Allowable bending stress(kgf/mm2) (See Figure 11.2)
f ：Speed factor (See Table 11.6)
Fig.11.2 Allowable bending stress σb
Table 11.5 Tooth profile factor y
Duracon
The allowable tangential force F(kgf) at the pitch circle of a Duracon 90 spur gear can be obtained from the Lewis formula.
F = mybσb (11.2)
Where
m ：Module(mm)
y ：tooth profile factor at pitch point (See table 11.5)
b ：Facewidth(mm)
σb ：Allowable bending stress(kgf/mm2)
The allowable bending stress σb can be obtained from:
Where σb：Maximum allowable bending stress under standard condition (kgf/mm2) See Figure 11.3
CS ：Working factor (See Table 11.7)
KV ：Speed factor (See Figure 11.4)
KT ：Temperature factor (See Figure 11.5)
KL ：Lubrication factor(See Table 11.8)
KM ：Material factor(See Table 11.9)
Fig.11.3 Maximum allowable bending stress under standard conditions, σb
Fig.11.4 Speed factor, KV
Fig. 11.5 Temperature factor, KT
Table 11.7 Working factor, CS
Table 11.8 Lubrication factor, KL
Table 11.9 Material factor, KM
Application Notes
In designing plastic gears, the effects of heat and moisture must be given careful consideration. The related problems are:
1. Backlash
Plastic gears have larger coefficients of thermal expansion. Also, they have an affinity to absorb moisture and swell. Good design requires allowance for a greater amount of backlash than for metal gears.
2. Lubrication
Most plastic gears do not require lubrication. However, temperature rise due to meshing may be controlled by the cooling effect of a lubricant as well as to reduce the of friction. Often, in the case of high-speed rotational speeds, lubrication is critical.
3. Plastic gear with a metal mate
If one of the gears of a mated pair is metal, there will be a heat sink that combats a high temperature rise. The effectiveness depends upon the particular metal, amount of metal mass, and rotational speed.
(2) Surface Durability of Spur Gears
Duracon
Duracon gears have less friction and wear in an oil lubrication condition. However, the calculation of durability must take into consideration a no-lubrication condition. The surface durability using Hertz stress, SC , (kgf/mm2) is calculated by Equation (11.4).
Where
F ：Tangential force on tooth(kgf)
b ：Facewidth(mm)
d01 ：Pitch diameter of pinion(mm)
i ：Gear ratio = z2/z1
E ：Modulus of elasticity of material (kgf/mm2) (See Figure 11.6)
α ：Pressure angle (degree)
Fig.11.6 Modulus of elasticity in bending of duracon
Fig.11.7 Maximum allowable surface stress – spur gears
If the value of Hertz contact stress, SC, is calculated by Equation (11.4) and the value falls below the curve of Figure 11.7, then it is directly applicable as a safe design. If the calculated value falls above the curve, the Duracon gear is unsafe. Figure 11.7 is based upon data for a pair of Duracon gears: m = 2, v = 12m/s, and operating at room temperature. For working conditions that are similar or better, the values in Figure 11.7 can be used.
(3) Bending Strength of Plastic Bevel Gears
MC Nylon
The allowable tangential force, F(kgf), at the pitch circle is calculated by Equation (11.5).
where
y ：Tooth profile factor at pitch point, which is obtained from Table 11.5 by first computing the number of teeth of equivalent spur gear, zv, using Equation (11.6).
zv = z / cos δ0 (11.6)
Ra ：Outer cone distance(mm)
δ0 ：Pitch cone angle(degree)
Other variables may be calculated the same way as for spur gears.
Duracon
The allowable tangential force, F(kgf), on pitch circle of Duracon bevel gears can be obtained from Equation (11.7).
y ：Tooth profile factor at pitch point, which is obtained from Table 1.5 by computing the equivalent number of teeth via Equation (11.6).
Other variables are obtained by using the equations for Duracon spur gears.
(4) Bending Strength of Worm Gear
MC Nylon
Generally, the worm is much stronger than the worm wheel. Therefore, it is necessary to calculate the strength of only the worm wheel.
The allowable tangential force F(kgf) at the pitch circle of the worm wheel is obtained from Equation (11.8).
Where
mn ：Normal module(mm)
y ：Tooth profile factor at pitch point, which is obtained from Table 11.5 by first computing the equivalent number of teeth, zv, using Equation (11.9).
Worm meshes have relatively high sliding velocities, which induces a high temperature rise. This causes a sharp decrease in strength and abnormal friction wear. Therefore, sliding speeds must be contained within recommendations of Table 11.10.
Table 11.10 Material combinations and limits of sliding speed
Sliding speed, vs, can be obtained from:
Lubrication is vital in the case of plastic worm gear pair, particularly under high load and continuous operation.
(5) Strength of Plastic Keyway
Fastening of a plastic gear to the shaft is often done by means of a key and keyway. Then, the critical thing is the stress σ (kgf/cm2) imposed upon the keyway sides. This is calculated by Equation (11.11).
T ：Transmitted torque(kgf・cm)
d ：Diameter of shaft(cm)
l ：Effective length of keyway(cm)
h ：Depth of keyway(cm)
The maximum allowable surface pressure of MC901 is 200kgf/cm2, and this must not be exceeded. Also, the keyway’s corner must have a suitable radius to avoid stress concentration. The distance from the root of the gear to the bottom of the keyway should be at least twice the tooth depth.
Keyways are not to be used when the following conditions exist:
– Excessive keyway stress
– High ambient temperature
– Large outside diameter gears
– High impact
When above conditions prevail, it is expedient to use a metallic hub in the gear. Then, a keyway may be cut in the metal hub.
A metallic hub can be fixed in the plastic gear by several methods:
– Press the metallic hub into the plastic gear, ensuring fastening with a knurl or screw.
– Screw fastened metal discs on each side of the plastic gear.
– Thermofuse the metal hub to the gear.
Related links (相关链接):
塑料齿轮的设计
Gear Design
Plastic Gears