Machinists attempting to restore or upgrade a 93 cm Bridgeport-pattern ballscrew face technical difficulties because official manufacturer data for these specific components is non-existent.
Hardinge, the company that acquired Bridgeport, holds no block or bearing data for the ballscrew featuring bare journals at both ends.
The Criticality of Manual Measurement
Before purchasing any hardware, the first step requires verifying the physical dimensions of the screw using micrometers and thread gauges.
The screw’s geometry presents several distinct features that must be confirmed individually. At End #1, the outboard 20 mm consists of an M25 x 1.5 thread, followed by a 73 mm middle section of 25 mm bare journal, and an inboard 30 mm section of M30 x 1.5 thread.
End #2 is more complex, featuring an outboard 40 mm stub of 16 mm diameter with two flats for drive interfaces, a middle 18 mm section of M25 x 1.5 thread, and an inboard 26 mm section of 25 mm bare journal.
A specific risk exists regarding how the bearings interface with these threads. If a bearing is intended to slide over the M25 x 1.5 thread to reach the journal, the technician must confirm the major diameter allows for this movement; otherwise, the stack cannot be assembled from the end.
Beyond diameter, the squareness of the journal shoulders is a primary concern for long-term stability. Using a dial indicator between centers to check journal runout and shoulder squareness is essential. A bearing pressed against an out-of-square shoulder will load unevenly, a problem that nut torque cannot fix.
Selecting a Mounting Configuration
The decision on how to hold the screw dictates the machine’s stiffness and its ability to handle thermal expansion. This choice effectively determines whether the machine’s lead error remains predictable or becomes nonlinear under fluctuating loads.
| Configuration | End #1 Setup | End #2 Setup | Mechanical Behavior |
|---|---|---|---|
| Fixed-free | DB or DF set | Floats | Simplest; screw becomes springier with distance from fixed end. |
| Fixed-supported | DB set | Radial bearing | Screw is not in tension; stiffness is lower than fixed-fixed. |
| Fixed-fixed | Thrust bearing(s) | Opposed thrust bearing(s) | Highest stiffness; requires precise pre-stretch calculations. |
While a fixed-fixed setup offers the highest stiffness, it introduces the complexity of pre-stretching the screw. For a 930 mm carbon steel screw, thermal growth can account for approximately 0.0107 mm per Kelvin. If the applied stretch does not exceed this expected growth, the screw may go slack, causing a direct loss in lead accuracy.
In a fixed-fixed setup, the two bearings work against each other to stretch the screw, meaning the load-line orientation must be read carefully from the bearing cross-section before installation.
Manage Spatial Constraints Within the 73 Mm Window
When sourcing off-the-shelf components, such as HIWIN 7602025TVP bearings, technicians must manage tight spatial constraints. These specific bearings have a 60-degree contact angle, providing high axial stiffness but lower radial capacity.
At End #1, the 73 mm bare journal section limits the available space for the bearing stack. Because the nuts sit on the threads, all bearing hardware must be contained within that 73 mm window.
For example, if using four 15 mm bearings, only 13 mm remains for spacers and washers. If two bearings are spread 40 mm apart, the total width of the bearings and the spread must stay under 73 mm to ensure a proper fit.
Precision in fastening is equally vital. Using a precision ring nut with locking grub screws on the M30 x 1.5 and M25 x 1.5 threads is recommended. The face of the nut must be square to the thread; any tilt will introduce runout into the bearing inner ring. The nut's diameter must not overlap the outer ring, cage, or seal of the bearing, which can cause mechanical dragging.
Troubleshooting Mechanical Symptoms
Common issues often point back to specific installation errors in the bearing or tensioning stages.
- Lead error changes with load: This often indicates fixed-free compliance, suggesting a move to a supported or fixed-fixed setup is required.
- Position drift as the screw warms: This typically results from insufficient stretch margin in a fixed-fixed setup or an End #2 bearing that is accidentally locked rather than floating.
- Clunking or axial play on reversal: This is frequently caused by unpaired single-direction bearings or loose nuts lacking proper preload.
- High turning torque when cold: This suggests excessive preload caused by spacer length or over-tightened nuts.
