An open belt and a crossed belt can select opposite directions on one driven shaft. A keyed centre pulley transmits torque; the outer loose pulleys let the unused belt continue running.
Belt-shifter Reversing Interactive Calculator
Compare belt width, actual shaft spacing and a selected spacing multiplier. Two animated configurations show which belt drives the centre pulley in each direction.
Equation Used
- Belt width and centre spacing are in inches.
- Open belts preserve rotation direction; crossed belts reverse it in the ideal no-slip model.
- The middle driven pulley is keyed; both outer pulleys run loose.
- Forward and reverse are separate configurations; shifting and neutral are not simulated.
Construction reference: Henry T. Brown, movement 2. The text describes a fast middle pulley and loose outer pulleys, with one belt driving at a time.
Choosing which belt drives
The open belt makes its driven pulley turn in the same direction as the driver. Crossing the other belt reverses its driven pulley. On the receiving shaft, the middle pulley is keyed to the shaft and the outer two can rotate independently.
In the forward configuration, the open belt occupies the centre pulley and the crossed belt runs on an outer loose pulley. In reverse, the crossed belt occupies the centre and the open belt runs loose. The two animations show these completed configurations separately. They do not imply that both opposing belts can drive the centre simultaneously.
The centre pulley is fixed to the shaft in Brown’s arrangement. The separate forward and reverse views show the resulting belt paths; the shifting transition and a neutral position are not simulated.
Historical reversing drives
This arrangement illustrates direction selection from a continuously rotating belt source. It is distinct from the one-belt reverser that uses concentric shafts and opposed bevel gears. Each arrangement needs its own mechanical connections and shifting provisions.
Spacing comparison
For belt width W and selected multiplier k, the reference centre distance is kW. The actual ratio is C/W. The signed difference C − kW shows whether actual spacing is above or below that reference. Percentage below reference is max(0, (kW − C)/(kW)) × 100.
The spacing multiplier does not establish acceptable tracking or belt life. The crossed-belt geometry in the illustration uses common internal tangents; its pulley wrap is larger than a half turn. Out-of-plane belt deformation and contact at the crossover are schematic.
Worked comparison
For W = 4 inches, k = 20 and C = 80 inches, the reference is 80 inches, the actual ratio is 20 times width and the difference is zero. With the same width and multiplier but C = 60 inches, the difference is −20 inches and the spacing is 25 percent below the selected reference. Neither result is a validated pass/fail limit.
Why two views are useful
The open and crossed paths are visible together, along with the fast and loose pulleys. Their opposite rotation is a geometric consequence of the belt route. The diagram omits engagement forces, belt slip, lateral tracking and wear; those cannot be inferred from centre distance alone.
Reversing-belt questions
Which pulley is fixed to the driven shaft?
The centre pulley. The two outer pulleys turn independently of that shaft.
Is there a neutral position in this animation?
No. It shows separate forward and reverse configurations. A practical neutral arrangement requires additional defined pulley and shifter geometry.
Is twenty times the belt width a guaranteed minimum spacing?
No. The multiplier is a dimension comparison. Belt construction, pulley geometry and tracking requirements need separate evaluation.
Original construction reference
Henry T. Brown, Five Hundred and Seven Mechanical Movements, movement 2: crossed-belt reversal and the open/crossed two-belt arrangement with a fast centre pulley and loose outer pulleys.
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