Durable Gear Pump with Different Teeth

A discharge zone is schematically shown on the picture. Direction of  turning resistance T0 is opposite to rotation.  Here S is the surface area of addendum cross section along gear axis, B - face width, m - module, da - addendum diameter, d - pitch diameter. We can consider Ft to be equal F0, because their action points have almost equal radii: Ft ~ F0 = PmB. If we define Wt as distributed circumferential force on unit width of teeth, than Wt = Pm. So, if the liquid pressure is 10N/sq.mm and the teeth module is 1mm, than Wt ~ 10N/mm. In case of similar gear pump, assuming that its teeth module equals to sliding big teeth module, Wt ~ 50N/mm, since the pitch ratio here is taken 5. It follows, that: circumferential force Wt is Pr times less than it is in case of similar gear pump under equal liquid pressures! 

Since the teeth friction forces are linearly dependent from circumferential force, they are also less than Pr times!

Now let's see the impact of teeth size on their bending stress and contact stress (pitting resistance) caused by circumferential force:

The bending stress will be: 

 Here, the YF is geometry factor, whereas KFb and KFv are load-distribution factor and dynamic factor for bending stress respectively. These 3 parameters have insignificant change depending on module and teeth number. So, we can say based on the above formula: 

the teeth bending stress does not depend on their module under equal liquid pressures!

Let's analyse contact stress:

 Here, ZH is teeth meshing surfaces shape factor, ZM - coefficient of materials mechanical properties, Ze - counts on contact ratio, KHb and KHv have similar meanings like KFb and KFv for contact stress. These parameters also have insignificant change depending on module and teeth number. Hence, it follows from the formula: 

In our case (Pr=5), the contact stress will be about 2.24 times smaller compared with gear pump having teeth module and pitch diameter equal to sliding teeth module and pitch diameter of this pump. 

The friction force and contact stress are main factors of teeth wear. The first one is responsible for depreciation and the other one - for contact fatigue. That is why their reduction significantly improves the pump durability and reliability! Besides, high number of small teeth reduce dynamic loads and make gear meshing smoother. Big sliding teeth cannot cause dynamic load, since the time interval, when meshing takes place only between big tooth and big cavity, is small (see animation) and rotating inertia of both gears will prevent dynamic load to arise here. Gear pumps got the worst grade among positive displacement pumps in terms of operational lifetime in Rexroth didactic. The proposed model will lift the pump position according to this criterion. 

The other great advantage of this pump is the possibility to make driven gear from plastics, such as polyamide and POM (durable metal-plastic gear pump)  

The gears made from these materials can withstand up to 20 N/mm distributed circumferential force. If they are reinforced with  fiberglass, carbon and aramid, they can withstand up to 40 N/mm load. So, if gear teeth have 1 - 2 mm module, the gear can withstand up to 20 N/sq.mm=200 Bar liquid pressure based on Wt = Pm formula. That's why it is possible to generate higher liquid pressure even if the driven gear is made of plastics. 

1. Sufficient hermeticity to carry low viscous liquids like water at higher pressure.  

   1. One of the disadvantages of gear pump is slippage or lost GPM  especially for low viscous liquids at higher pressure. Since here plastic teeth have elastic contact with metal teeth, slippage through meshed teeth is practically eliminated, which will increase the pump volumetric efficiency.

2. The pump can handle suspended solids or abrasives.

• 1. The other disadvantage of gear pump is accelerated wear of teeth flank in the presence of abrasives. This problem is resolved here, because abrasives will enter into plastic teeth during meshing and then will be washed out. Polyamides, for instance, are abrasive resistant. Sleeve bearings, made of it, work well in submersible pumps at dirty water.

3. Long lifetime and high reliability   

• 1. Here the lifetime is improved even more, because metal to plastic teeth contact surface is larger, resulting low contact pressure, which significantly reduces wear rate of metal gear. The plastic gears are temperature sensitive, since their mechanical properties drop depending on teeth surface temperature. However, this issue is not important here, because the pumped liquid both cools and lubricates (most of the time) plastic gear teeth. It is worth to mention that plastic gears have vibration dampening property, which obviously contributes to pump lifetime.

4. Low noise

• 1. Steel against steel gears generate up to 3 times as much noise than do steel against plastic gears. 

5. Low cost

6. Plastic gears are manufactured with the same machining process as are metal gears, such as milling or hobbing. Since cutting forces here are much less, it is possible to do machining much quicker with high infeed rates. Besides, wider tolerances can be allowed due to elasticity of the plastic. Taking into consideration also the lower price of plastic material, we can surely say that plastic gear manufacturing cost is significantly lower than metal one. 

7. The cost of manufacturing coupled driving metal gear is lower too, since here is also no need for tight tolerances and high surface quality of teeth flanks. Even, high surface quality has negative influence on the coupled plastic gear. The recommended range of surface roughness is 0.9 -1.2 Ra. So, the teeth grinding operation can be omitted! If premachined metal is hardened up to 40 HRC, it can be further milled or hobbed to get the mentioned surface roughness.  

Of course, plastic gears currently are used in gear pumps, but compared with the suggested pump, they cannot generate high pressure, run smoother (because of low engagement factor), have high suction lift and other advantages listed in the home page of my website. 

Conclusion

After brief analysis both in this page and in the home page of my website, we come in conclusion, that "durable gear pump with Different Teeth" has the following advantages compared with gear pump:

The last point permits to use this pump for delivering low flow water at high pressure in applications like pressure washing, power jet irrigation and in water supply of high discharge head, such as pumping from wells.