One shifting belt selects forward, neutral or reverse motion of an upright shaft. Two opposed bevel gears are carried by concentric horizontal shafts; the centre pulley runs loose. This is the arrangement in Henry T. Brown’s movement 7.
Belt Engage Disengage Reverse Interactive Calculator
Compare belt width, adjacent pulley spacing, fork clearance and the entered detent force. Watch one belt select either shaft or the loose centre pulley.
Equation Used
- Equal pulley face widths; adjacent centres separated by one face width plus the entered gap.
- Belt width is in inches; gap and each-side fork clearance are in millimetres.
- Three-pulley selection and equal bevel gears are illustrative.
- No shift-force, slip, tooth-strength or dynamic engagement calculation.
Construction reference: Henry T. Brown, Five Hundred and Seven Mechanical Movements, movement 7. One belt selects the loose centre pulley or either of the two concentric driven shafts.
One belt, two concentric shafts
The left driven pulley is fast on a hollow shaft carrying bevel gear B. The right driven pulley is fast on the inner shaft carrying bevel gear A. Between them, the loose pulley can turn without driving either shaft.
Both opposed bevel gears mesh with the upright output gear C. Driving B in one direction turns C one way. Driving A in that same direction turns C the opposite way. The unselected side gear and its pulley turn backwards through the gear train. This is why the concentric shafts must turn independently.
In the animation, the upper driver continues rotating while the belt shifts between the three positions. Neutral is shown stationary at the output; an actual output may coast. Engagement transients and belt slip are not simulated.
Historical purpose
This arrangement let a continuously running belt source select either direction of an upright shaft. It illustrates mechanical direction selection using a loose pulley and opposed bevel gears. It differs from the open-and-crossed-belt reversing drive, which uses two belts.
Layout calculations
The reference face width is 1.25 times belt width B. This allowance provides a geometric comparison; the required face width depends on the actual belt and pulley design. For equal pulley faces with axial gap g, the adjacent centre spacing is 25.4 × face width in inches + g in millimetres. The full left-to-right traverse is twice the adjacent spacing.
A fork with clearance c on each belt edge has an inside opening of 25.4B + 2c millimetres. The entered detent force is converted with 1 lbf = 4.448221615 N. These dimensions do not calculate the force needed to shift a running belt.
Worked layout example
For a 4-inch belt, the reference face is 5 inches or 127 mm. A 3 mm gap gives 130 mm between adjacent pulley centres and 260 mm between the two outer positions. With 5 mm clearance on each edge, fork opening is 111.6 mm. An entered 15 lbf detent force equals 66.72 N.
What the model does and does not show
The concentric shafts and opposed bevel gears make the reversing principle visible. The view simplifies bearings and shows the gears as pitch-cone illustrations with tooth markings. Tooth form, crowning, stiffness, guards and shifting hardware need their own detailed design. The calculator does not provide a fit score or a safety rating.
Questions about the reversing drive
Why does the unused outer pulley turn backwards?
It remains connected through its bevel gear to the upright output. Only the central loose pulley is independent of both driven shafts.
Does neutral stop the output immediately?
No. It removes the driving connection. This explanatory animation holds the output during neutral and omits coasting dynamics.
Does the detent-force result tell me the needed shifter force?
No. It converts the entered force from pounds-force to newtons. The required shifting force depends on the belt, fork, support friction and operating conditions.
Original construction reference
Henry T. Brown, Five Hundred and Seven Mechanical Movements, movement 7: one belt shifted between a loose middle pulley and two pulleys attached to concentric shafts carrying opposed bevel gears.
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