Piston Rod Arrangement Eliminating Bung Clearance
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Solution Overview
Problem
Drug delivery devices, particularly pen-type injection devices, face a technical challenge where the first dose delivered is often less than the set dose due to a gap between the piston rod and the bung, resulting in reduced accuracy and requiring a priming step like 'air shot' to initiate drug ejection.
Innovation Solution
The device incorporates a piston rod arrangement with a drive member that is threadedly engaged with both the piston rod and a body member, featuring a mechanism to eliminate clearance between the piston rod and the bung, ensuring the piston rod is in contact or at a precise distance from the bung before use, eliminating the need for a priming step and enhancing dose accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If a gap is left between the piston rod and the bung due to manufacturing tolerances, then the device can be assembled without pre-loading the bung, but the first dose delivered will be less than the set dose, reducing dose accuracy
Solution Approach 1:
The patent applies preliminary action by pre-positioning the piston rod at a defined distance from the bung during assembly, rather than allowing a gap to form. The piston rod is initially positioned at a distance corresponding to one unit step of movement, and the mechanism is designed so that the first actuation eliminates the gap and delivers the correct dose without requiring a priming step. This preliminary positioning resolves the contradiction by ensuring dose accuracy from the first use while maintaining ease of manufacture through a standardized assembly process.
2Measurement precision
If the piston rod is positioned in contact with or at a precise distance from the bung before first use, then dose accuracy is improved, but a priming step is eliminated and the device is ready for immediate accurate use
Solution Approach 1:
The invention applies preliminary action by pre-configuring the piston rod position during manufacturing at a distance of one unit step from the bung. This preliminary positioning ensures that when the device is first used, the mechanism eliminates any gap and delivers the full set dose accurately without requiring a priming step. The time loss associated with priming is eliminated because the piston rod starts in the correct position, ready for immediate accurate dosing.
3Measurement precision
If the piston rod is positioned at a distance less than one unit step from the bung, then dose accuracy is enhanced, but the mechanism complexity increases to define precise unit steps
Solution Approach 1:
The patent applies dynamics by designing a mechanism where the piston rod position is dynamically adjusted during the first actuation. The drive member and piston rod are configured so that the first rotation eliminates the gap between the piston rod and bung, and subsequent rotations deliver precise unit doses. This dynamic adjustment during initial operation achieves high dose accuracy without requiring complex pre-adjustment mechanisms, resolving the contradiction between precision and complexity.
Solution Approach 2:
The invention applies segmentation by dividing the dose delivery mechanism into discrete unit steps. The drive member is configured with features that define unit steps of rotation, which correspond to unit steps of piston rod advancement. This segmentation allows the piston rod to be positioned at a precise distance (one unit step) from the bung, enabling accurate dose delivery while using a relatively simple mechanism based on discrete rotational steps rather than continuous adjustment.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution ensures that the first dose delivered is accurate and within allowable limits without the need for a priming step, maintaining the integrity of the drug volume and improving user experience by eliminating the initial dose reduction.
Implementation Method 1
The piston rod is threadedly engaged with the body member, and the drive member is threadedly engaged with the piston rod and movable relative to the body member
Data Source
Figure 1
Figure 1a
Figure 2~3
AI summary
The piston rod arrangement is in an assembled state before use and comprises a piston rod (1 ), a bung (2), which is intended to be driven by the piston rod, a body member (5), and a drive member (6). The piston rod is threadedly engaged with the body member, and the drive member is threadedly engaged with the piston rod and movable relative to the body member. A mechanism is provided defining unit steps of movement of the drive member (6) and corresponding unit steps of movement of the piston rod (1 ). The piston rod is arranged in contact with the bung or at a distance from the bung that is less than a distance of one corresponding unit step of movement. When the piston rod, the bung and the drive member are assembled in the body member, the piston rod is advanced with respect to the body member towards the bung, until the piston rod is stopped by the bung. The drive member is rotated until the piston rod begins to advance again and the drive member is then stopped, and the piston rod is fixed at or near the position obtained.