A hook-and-knife dobby selects which heald shafts move for each loom pick. Explore the selection mechanism, then convert a chosen crank-angle interval to elapsed time at an entered pick rate.
Dobby Pick-Period and Angular-Window Converter
Enter loom speed and two user-defined crank-angle windows. The results are time conversions only; they do not establish a machine's actual selection window, clearance, reliability or safe operating speed.
Equation Used
- One pick cycle corresponds to one full 360-degree reference cycle.
- The entered speed is constant and the two angles are user-defined comparison windows.
- Hook motion, knife motion, clearances, shaft dynamics, controller delay, loads and manufacturer limits are excluded.
Selected hooks engage the moving knife; bypassed hooks miss its stroke. The separate angle display converts user-defined portions of a pick cycle into milliseconds, without setting real engagement or dwell limits.
Hook-and-knife dobby topology
A pattern source controls whether each hook is placed in the path of a reciprocating knife. When selected, the knife carries the hook and moves a connected lever or jack that operates a heald shaft. A bypassed hook does not receive that knife stroke. Double-lift dobbies use alternating sets so shafts can remain raised or lowered across successive picks.
The animation is a labelled topology sketch. It alternates two illustrative channels to make selection and bypass visible; it is not a timing diagram or dimensional model of a particular dobby.
Use as a unit conversion, not a machine setting
The calculator converts any independently known crank-angle interval into elapsed time at an entered pick rate. Determine actual selection, engagement, dwell and motion intervals from the exact loom and dobby documentation.
Angle-to-time conversion
T is milliseconds per pick for N picks/min. An angle theta occupies the same fraction theta/360 of that period. This identity does not reveal what event occurs during that angle.
Arithmetic example
At 600 picks/min, one pick lasts 100 ms. User-defined angles of 108 and 180 degrees therefore correspond to 30 and 50 ms. Those values are conversions only, not asserted selection windows or acceptance limits.
Model boundary and safety
This page does not predict engagement, missed picks, warp damage, forces, wear, vibration or safe speed. Follow the exact manufacturer timing, clearance, guarding, lockout and maintenance instructions.
Dobby timing questions
Does a longer converted time prove reliable selection?
No. Reliability depends on the actual event timing and complete machine dynamics.
Do angles A and B have to be minimum and maximum dwell?
No. They are simply user-defined intervals for comparison.
Does the drawing show a modern rotary dobby?
No. It identifies a historical hook-and-reciprocating-knife topology.
Primary and museum references
- Science Museum Group, Hand loom with dobby mechanism: documents lag pegs selecting vertical hooks, a horizontal knife carrying selected hooks and heald shafts hanging from the hooks.
- US Patent 1,739,194, Loom dobby mechanism: describes harness jacks, upper and lower dobby hooks, sliding knife bars and pattern-chain selection.
- EP Patent 0,071,942, Knife oscillations and hook movements in a loom dobby: identifies dobbies as loom shedding apparatus and describes coordinated knife, hook and linkage motion.
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