How to calculate the required length of an axial pump piston

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Discussion Overview

The discussion revolves around calculating the required length of a piston in an axial piston pump, focusing on the parameters that influence this measurement. Participants explore various factors that affect piston length, including engineering judgments and optimization considerations.

Discussion Character

  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant inquires about the relevant equations for determining piston length, noting that they have found many for diameter but not for length.
  • Another participant suggests considering the required difference in volume between the piston in the up and down positions.
  • It is proposed that piston length is influenced by factors such as the maximum angle between the connecting rod and bore centerline, peak force on the piston, wrist pin location, and allowable bearing stress. There is no single equation for this calculation.
  • Clarification is sought regarding whether the discussion pertains to piston length or stroke, with confirmation that the focus is indeed on length.
  • A participant emphasizes that this problem is not typical and involves optimization with multiple potential solutions, depending on various factors like size, friction, parts count, cost, and reliability.
  • Further questions are raised about additional parameters such as the number of piston rings, wrist pin diameter, axial alignment of the piston, and the effects of heat on the piston when pumping compressed air.
  • Concerns are expressed about the skirt of the piston needing to be clear of the crankshaft, suggesting that longer skirted pistons require longer connecting rods and bores.

Areas of Agreement / Disagreement

Participants express differing views on the complexity of the problem, with some agreeing on the need for engineering judgment and optimization, while others highlight the lack of a single solution. The discussion remains unresolved with multiple competing perspectives on how to approach the calculation.

Contextual Notes

Participants note that the problem requires iterative calculations and depends on various assumptions and design choices, which are not fully defined in the discussion.

ENGBIO
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Based on other parameters how do you calculate the required length of a piston inside the cylinder block of an axial piston pump?

What are the relevant equations? I've found many for the required diameter but not the length.
 
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How about the required difference in volume, piston up and piston down?
 
The piston length is determined by the maximum angle between the connecting rod and the bore centerline, the peak force on the piston, the location of the wrist pin, and the allowable bearing stress on the side of the piston. There is no single equation that I am aware of. You need to use engineering judgement to determine the rod length ratio and allowable bearing stress, then calculate from there. The calculations are straight from Statics 101.
 
Do you mean length or stroke?
 
CWatters said:
Do you mean length or stroke?
No i meant length
 
Then I agree with post #3.
 
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This is not a typical academic problem with a single exact solution. It is an optimization problem with many solutions, none of which are "the best". You can optimize for least overall size, lowest friction, least number of parts, lowest cost, highest reliability, etc, etc. You choose how you weight the various factors using engineering judgement, then do the calculations. It's an iterative process.
 
ENGBIO said:
how do you calculate the required length of a piston inside the cylinder block
How many piston rings are there?
What is the diameter of the wrist pin?
What keeps the piston axially aligned with the cylinder?
What are you pumping? The heat of compressed air will expand the piston unless the skirt/sides can cool by radiating heat into the block.
Skirt of piston must be clear of crankshaft so connecting rod and bore must be longer for longer skirted pistons.
 

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