A Crayford focuser moves a telescope drawtube with a smooth drive shaft rather than gear teeth. The shaft presses the tube against four support rollers. Turning the shaft translates the tube, while the roller arrangement guides it.
Crayford Focuser Interactive Calculator
Compare ideal shaft traction with axial payload weight. Rotate the installation and inspect the smooth drive and four-bearing guide arrangement.
Equation Used
- One smooth shaft-to-tube driving contact.
- Entered total normal contact force and constant friction coefficient.
- Free-running support rollers are not added as another drive contact.
- Static axial gravity only; drive torque, friction losses and payload moments are not evaluated.
- Unloaded kinematic preview with illustrative travel and dimensions.
One driven contact: ideal traction is μN. Motion is an unloaded preview, not proof that the entered payload can be lifted.
Smooth shaft and guided drawtube
The drive shaft contacts the tube’s flat surface at one driven friction contact. Two pairs of freely turning bearings support the opposite side of the drawtube in a V arrangement. The side section shows one bearing row; the end view shows both rows.
The illustration rolls the shaft and support bearings consistently with the tube’s translation. Changing tube angle rotates the installation while the gravity direction remains fixed. Motion is an unloaded preview: it is not a prediction that an overloaded focuser will lift its payload.
Checking axial payload traction
Use the entered contact friction coefficient and total shaft-to-tube normal force to compare ideal available traction with the payload’s gravity component along the tube. The calculation can identify an inadequate ideal traction margin, but it is not a focuser’s rated payload.
One driven contact
The ideal tangential traction limit is F=μN, where N is the total normal force at the one drive-shaft contact. Free-running support bearings do not create a second driven friction surface, so this expression is not doubled.
For tube angle α above horizontal, the axial payload force is mg sinα. The traction-to-load ratio is μN/(mg sinα). At zero angle the axial gravity component is zero, so the traction-to-axial-load ratio has no finite value. Equivalent vertical mass is μN/g.
Example contact load
With μ=0.18 and shaft contact force N=70 N, ideal available traction is 12.6 N. A 1.5 kg payload on a vertical tube imposes about 14.71 N axially. The ratio is about 0.86, so this ideal calculation does not provide enough traction for that payload.
At 30 degrees above horizontal, the axial gravity component is about 7.35 N. The same contact then gives a ratio near 1.71. This comparison still excludes rolling resistance and the torque needed to move the shaft.
What a traction calculation leaves out
The normal force is the contact load, not automatically a tension-screw force or spring preload. Their relationship depends on the bracket and bearing geometry. Friction coefficient also depends on the actual surfaces and conditions.
This simplified model excludes bearing resistance, structural flex, misalignment, acceleration, drive torque and payload moments. It does not establish zero backlash, a universal surface finish, a material hardness requirement or a safe adjustment setting.
Focuser questions
Why are there four support bearings but only one drive contact?
The bearings guide the tube and turn freely. The smooth shaft supplies the driving traction at its own contact with the tube.
Why does the illustration move when the load exceeds traction?
It is an unloaded kinematic preview. The status explicitly reports that the entered payload exceeds the ideal traction limit.
What angle should I enter?
Enter the drawtube axis angle above horizontal. Vertical is 90 degrees.
Is the traction ratio a certified safety factor?
No. It compares one ideal traction estimate with the axial gravity load; other operating effects remain outside the model.
Reference
Crayford Manor House Astronomical Society: Crayford Eyepiece Mount, describing John Wall’s smooth-pinion drive and V-arranged support rollers. The focuser photograph shows the smooth drive shaft and support-roller arrangement. The calculator’s force comparison follows the one-contact Coulomb-friction model.
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