Screw Accuracy
Haydon Kerk uses a unique precision rolling process for screw manufacturing. Standard lead accuracy for Kerk screws is .0006 in./in. (mm/mm). Lead accuracies are available up to .0001 in./in. (mm/mm). Please consult the factory for higher lead accuracies. Assemblies have an extremely high bi-directional repeatability of 50 micro-inches (1.25 micron).
End Machining
Haydon Kerk can custom machine screws to your specifications or provide cut-to-length screws for your own machining.
Critical Speed
This is the rotational speed at which a screw may experience vibration or other dynamic problems. See CRITICAL SPEED CHART to determine if application parameters result in speed approaching critical. To minimize critical speed problems: use a longer lead, choose a larger diameter or increase bearing mount support.
Lengths
Lengths can be specified up to 12 ft. (4M) from stock, (depending on diameter and lead). Cut to length screws are offered in 6-in increments (6-in, 12, 18,....) +1.0-in/-0-in.
Lead
Advancement per revolution. All screws are listed by lead, not pitch. Lead = Pitch x Number of Starts
Pitch
Crest-to-crest distance or one divided by threads per inch. (On a multiple start thread, the pitch equals the lead divided by the number of starts.)
Traverse Speed
The nut materials we use provide long wear-life over a wide variety of conditions. However, very high loads and/or speeds will accelerate nut wear. Special materials may be required for these situations. We offer the following guidelines for continuous duty linear traversing speeds
for optimum life:
| Lead |
Traverse Speed |
Lead |
Traverse Speed |
| 1/10 - 1/2-in |
4-in/sec. |
1 - 12 mm |
100 mm/sec. |
| 1/2 - 1-in |
10-in/sec. |
12 - 25 mm |
250 mm/sec. |
| 1 - 2 1/2-in |
30-in/sec. |
25 - 60 mm |
760 mm/sec. |
Maximum Load
Although the Kerk® Anti-Backlash Assemblies are capable of withstanding relatively high loads without catastrophic failure, these units have been designed to operate under the loading shown in the size charts.
Efficiency
Efficiency is the relationship of work input to work output. It should not be confused with mechanical advantage. Listed efficiency are theoretical values based on Kerkote® TFE coated screws.
Torque
The required motor torque to drive a lead screw assembly is the sum of three components: the inertial torque, drag torque, and torque-to-move load. It must be noted that this is the torque necessary to drive the lead screw assembly alone. Additional torque associated with driving frictional bearings and motor shafts, moving components, and drag due to general assembly misalignment must also be considered.
Inertial Torque:
T = I α Where I = screw inertia
j α = angular acceleration
Drag Torque:
Kerk Anti-Backlash Assemblies are typically supplied with drag torque of 1 to 7 oz.-in. The magnitude of the drag torque is dependent on the standard factory settings or settings specified by the customer. Generally, the higher the preset force, the better the Anti-Back-lash characteristics.
Torque-to-Move:
T=
LOAD x LEAD
L 2 π
x EFFICIENCY
Back Driving:
Sometimes referred to as reversibility, back driving is the ability of a screw to be turned by a thrust load applied to the nut. Generally, back driving will not occur when the screw lead is less than 1/3 the diameter for uncoated screws or 1/4 the diameter for Kerkote TFE coated screws. For higher leads where back driving is likely, the torque required for holding a load is:
T = LOAD x LEAD x BACKDRIVE EFFICIENCY
b 2π
Screw Straightness:
Screw straightness is measured as Total Indicator Runout (TIR). The standard straightness for lead screws is .003-in/ft. Haydon Kerk can provide tighter specifications per request.